From f3bc24b638c416cb25bc25682329e43944df68be Mon Sep 17 00:00:00 2001
From: Mikhail Yenuchenko <public@mixeme.ru>
Date: Mon, 3 Aug 2026 00:29:14 +0300
Subject: [PATCH] =?UTF-8?q?feat:=20implement=20C.4=20=E2=80=94=20hermetic?=
 =?UTF-8?q?=20container=20e2e=20gate=20+=20native=20release=20build?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Separate test/e2e Go module drives the shipped deploy/docker-compose.yml
(plus a test-only override: self-signed cert, low ports, isolated compose
project) against a fake DNS zone (CoreDNS) and an smtp-sink MX, exactly as
an administrator and their applications would over HTTP/SMTP — covering the
class of failure unit tests can't see (container wiring). Positive path:
setup -> login -> domain -> DKIM record published into the fake zone ->
application -> SMTP AUTH send -> DKIM verified against the DNS-published
key -> send-log queued->sent. Negative: no-AUTH/unauthenticated relay,
sender/login mismatch, L1 (anvil) and L2 (panel) rate limits, journal-milter
fail-open, SELFPOST_HOSTNAME gate, session survives docker restart.

release.yml moves off qemu to a native per-arch build (amd64/arm64), each
gated by this suite before its tag is pushed and merged into the version
manifest.

Verified green on selfpost.example.com via `make e2e`; go vet/gofmt clean in
both modules.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
---
 .github/workflows/release.yml  |  86 +++++++++---
 .gitignore                     |   4 +
 CHANGELOG.md                   |  19 +++
 Makefile                       |  12 +-
 docs/progress.md               |   3 +-
 test/e2e/compose.override.yml  |  82 ++++++++++++
 test/e2e/dns/Corefile          |  16 +++
 test/e2e/dns_zone.go           | 120 +++++++++++++++++
 test/e2e/go.mod                |   7 +
 test/e2e/go.sum                |   4 +
 test/e2e/hostname_gate_test.go | 101 ++++++++++++++
 test/e2e/mail_helpers.go       |  86 ++++++++++++
 test/e2e/main_test.go          | 236 +++++++++++++++++++++++++++++++++
 test/e2e/negative_test.go      | 224 +++++++++++++++++++++++++++++++
 test/e2e/panel_client.go       | 234 ++++++++++++++++++++++++++++++++
 test/e2e/process_check.go      |  61 +++++++++
 test/e2e/sink/Dockerfile       |  26 ++++
 test/e2e/smtp_client.go        | 142 ++++++++++++++++++++
 test/e2e/stack.go              | 113 ++++++++++++++++
 test/e2e/stage.go              |  86 ++++++++++++
 test/e2e/wait.go               |  29 ++++
 21 files changed, 1674 insertions(+), 17 deletions(-)
 create mode 100644 test/e2e/compose.override.yml
 create mode 100644 test/e2e/dns/Corefile
 create mode 100644 test/e2e/dns_zone.go
 create mode 100644 test/e2e/go.mod
 create mode 100644 test/e2e/go.sum
 create mode 100644 test/e2e/hostname_gate_test.go
 create mode 100644 test/e2e/mail_helpers.go
 create mode 100644 test/e2e/main_test.go
 create mode 100644 test/e2e/negative_test.go
 create mode 100644 test/e2e/panel_client.go
 create mode 100644 test/e2e/process_check.go
 create mode 100644 test/e2e/sink/Dockerfile
 create mode 100644 test/e2e/smtp_client.go
 create mode 100644 test/e2e/stack.go
 create mode 100644 test/e2e/stage.go
 create mode 100644 test/e2e/wait.go

diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 6926f90..b64c5c9 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -6,26 +6,87 @@ name: release
 # into both the image tag and the panel binary's -ldflags version, so the two
 # can never drift apart (the invariant restore's version check in spec 7.5.A
 # depends on).
+#
+# Native per-architecture builds (plan implementation-plan.md C.4), not qemu:
+# running the full Postfix/OpenDKIM stack under emulation for the e2e gate
+# below is impractically slow. Each arch builds, e2e-gates and pushes its own
+# tag on its own native runner; a merge job then combines them into the one
+# manifest tag documented in deploy/docker-compose.yml. "test, then push" (not
+# push-by-digest then test) is deliberate: it means the bytes that get tagged
+# are exactly the bytes that passed e2e, at the cost of per-arch tags lingering
+# in the registry as a side effect (harmless — the version tag's immutability,
+# spec 10.1, is about that tag, not these).
 on:
   push:
     tags:
       - "v[0-9]+.[0-9]+.[0-9]+"
+  workflow_dispatch:
 
 permissions:
   contents: read
   packages: write
 
 jobs:
-  build-and-push:
+  prepare:
     runs-on: ubuntu-latest
+    outputs:
+      version: ${{ steps.version.outputs.version }}
     steps:
-      - uses: actions/checkout@v4
-
       - name: Derive version from tag
         id: version
         run: echo "version=${GITHUB_REF_NAME#v}" >> "$GITHUB_OUTPUT"
 
-      - uses: docker/setup-qemu-action@v3
+  build:
+    needs: prepare
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - runner: ubuntu-latest
+            arch: amd64
+          - runner: ubuntu-24.04-arm
+            arch: arm64
+    runs-on: ${{ matrix.runner }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Build image (native, loaded locally for the e2e gate)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: build/Dockerfile
+          load: true
+          provenance: false
+          build-args: |
+            VERSION=${{ needs.prepare.outputs.version }}
+          tags: selfpost:e2e
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.26"
+          cache-dependency-path: test/e2e/go.sum
+
+      - name: e2e (gates publishing — see docs/implementation-plan.md C.4)
+        run: cd test/e2e && go test -v -timeout 20m ./...
+
+      - name: Log in to ghcr.io
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Push per-arch tag
+        run: |
+          docker tag selfpost:e2e "ghcr.io/${{ github.repository }}:${{ needs.prepare.outputs.version }}-${{ matrix.arch }}"
+          docker push "ghcr.io/${{ github.repository }}:${{ needs.prepare.outputs.version }}-${{ matrix.arch }}"
+
+  merge:
+    needs: [prepare, build]
+    runs-on: ubuntu-latest
+    steps:
       - uses: docker/setup-buildx-action@v3
 
       - name: Log in to ghcr.io
@@ -35,14 +96,9 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Build and push
-        uses: docker/build-push-action@v6
-        with:
-          context: .
-          file: build/Dockerfile
-          platforms: linux/amd64,linux/arm64
-          build-args: |
-            VERSION=${{ steps.version.outputs.version }}
-          push: true
-          provenance: false
-          tags: ghcr.io/${{ github.repository }}:${{ steps.version.outputs.version }}
+      - name: Merge per-arch tags into the version manifest
+        run: |
+          docker buildx imagetools create \
+            -t "ghcr.io/${{ github.repository }}:${{ needs.prepare.outputs.version }}" \
+            "ghcr.io/${{ github.repository }}:${{ needs.prepare.outputs.version }}-amd64" \
+            "ghcr.io/${{ github.repository }}:${{ needs.prepare.outputs.version }}-arm64"
diff --git a/.gitignore b/.gitignore
index 143871f..b204197 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,3 +7,7 @@ dev/
 
 # Local Go caches / test output, if ever present.
 *.out
+
+# e2e scratch stand (generated cert, DKIM zone, sink dump, /data bind mount)
+# — recreated fresh by every `make e2e` run, never checked in.
+/test/e2e/.stage/
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2df2907..8223b03 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,25 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+- ci: hermetic container e2e suite (`test/e2e`, a separate Go module) gates
+  image publishing — `make e2e` locally, and `go test ./...` in `test/e2e` as
+  a required step in `release.yml` before a version tag's image is pushed.
+  It builds the real image, brings up the shipped `deploy/docker-compose.yml`
+  plus a test-only override (self-signed cert, low ports, a fake DNS zone
+  served by CoreDNS, a `smtp-sink` sink-MX) on an isolated compose project,
+  then drives the panel over HTTP exactly like an administrator: setup →
+  login → add a domain → publish the DKIM record it prints into the fake zone
+  → add an application → send over SMTP AUTH → verify the delivered message's
+  DKIM signature against the record the panel published → poll the send log
+  to `sent`. Negative coverage: no-AUTH and unauthenticated-relay rejection,
+  sender/login mismatch, the level-1 (anvil) and level-2 (panel-configured)
+  rate limits, the journal-milter's fail-open behaviour when the panel process
+  is stopped, a missing/malformed `SELFPOST_HOSTNAME` failing the container
+  fast, and a login session surviving `docker restart`. `release.yml` moved
+  off qemu to a native per-architecture build (`ubuntu-latest` /
+  `ubuntu-24.04-arm`), each gated by this suite before its tag is pushed and
+  merged into the version manifest — running the full Postfix/OpenDKIM stack
+  under emulation for the gate was impractically slow.
 - ops: `mail.log` rotation switched from `copytruncate` to rename +
   `postfix reload` (the same mechanism `postfix logrotate` itself uses),
   eliminating the up-to-one-second window in which `copytruncate` could drop
diff --git a/Makefile b/Makefile
index 42e3305..5ac55d0 100644
--- a/Makefile
+++ b/Makefile
@@ -13,7 +13,7 @@ MODULE  := codeberg.org/mix/selfpost
 LDFLAGS := -X $(MODULE)/internal/buildinfo.Version=$(VERSION)
 GOFLAGS := -trimpath
 
-.PHONY: all build vet test clean
+.PHONY: all build vet test clean e2e
 
 all: vet build
 
@@ -29,3 +29,13 @@ test:
 
 clean:
 	rm -rf bin
+
+# Hermetic container e2e (plan implementation-plan.md C.4): separate Go module
+# under test/e2e so its test-only dependencies (DKIM verification) never enter
+# this module's build graph. Builds the image fresh from this checkout, brings
+# up deploy/docker-compose.yml plus a test-only override on high ports and an
+# isolated compose project (-p selfpost-e2e) so it never collides with a real
+# deployment on the same host, then tears the stand down whether the suite
+# passed or not.
+e2e:
+	cd test/e2e && go test -v -timeout 20m ./...
diff --git a/docs/progress.md b/docs/progress.md
index 61cb5c9..778bd10 100644
--- a/docs/progress.md
+++ b/docs/progress.md
@@ -41,7 +41,8 @@
 - **B.1 реализован** (не выкачен на прод): сессии переехали в SQLite (`internal/store/migrations/0002_sessions.sql`, `internal/store/sessions.go`, `internal/web/session.go`) — хранится SHA-256 токена, не сам токен; скользящий срок бездействия `PANEL_SESSION_IDLE_DAYS` (по умолчанию 7 дней, без абсолютного потолка); запись в БД продлевается не чаще раза в час (`renewThreshold`); опросы мониторинга (`GET` с `HX-Request`) продление не триггерят (`isSessionActivity` в `internal/web/middleware.go`); `Max-Age` cookie выставляется тем же значением при логине и при продлении (`setSessionCookie`); смена пароля разлогинивает все сессии кроме текущей (уже было, теперь через БД). Проверено на стенде: логин → рестарт процесса панели → сессия жива по старой cookie; HX-Request-опрос и повторный GET внутри часового окна не шлют `Set-Cookie`. `go vet`/`go test ./...`/`gofmt -l .` чистые.
 - **B.2 реализован** (не выкачен на прод): ротация `mail.log` ушла с `copytruncate` на «переименовать + `postfix reload`» — `build/logrotate-mail.conf` (`nocreate` заменён на `create 0644 root root` **не по плану, а по стендовой проверке**: после reload Postfix пересоздаёт лог сам только в момент следующей фактической записи и с режимом `0600`, недоступным непривилегированной панели, — `create` в logrotate закрывает это, отдавая файл ей же на 644 сразу после переименования); `follow()` в `internal/logtail/logtail.go` при обнаружении смены inode дочитывает старый дескриптор ещё раз перед переключением; `readLogTail()` в `internal/web/handlers_monitor.go` считает отсутствующий файл пустым экраном, а не ошибкой. Проверено на стенде (`selfpost.example.com`, отдельный контейнер `selfpost:b2test2`): цикл трафик → принудительная ротация → файл пуст и сразу читаем непривилегированным uid панели (0 читает `mail.log` сразу после rename, без окна недоступности) → новый трафик после ротации уходит в новый файл на 644, ничего не потеряно по обе стороны rename. `go vet`/`go test ./...`/`gofmt -l .` чистые (на dev-сервере; локально на Windows `TestFollowTailsAndRotates` падает — rename открытого файла запрещён ОС, к делу не относится).
 - **B.3 реализован** (не выкачен на прод): `build/entrypoint.sh` проверяет `SELFPOST_HOSTNAME` до `postfix-config.sh` и до `supervisord` — при пустом значении `exit 1` с развёрнутым текстом ошибки (что это за имя, почему обязательно, пример, где задаётся); плюс синтаксическая проверка через `case`: минимум одна точка, без схемы/порта/пробелов (`*://*`, `*:*`, пробел/таб — тот же класс тихого спам-отказа, что и пустое значение). `saslRealm()` и fallback в `postfix-config.sh` не тронуты — после гейта эти ветки мертвы. Заодно отмечена обязательность переменной в `README.md` и `deploy/.env.example`. Проверено на стенде (`selfpost.example.com`, отдельный образ `selfpost:b3test`, cap-list как в поставляемом compose): без переменной — `exit 1` с ожидаемым текстом, без бесконечного тихого retry; `https://mail.example.com:465` и `localhost` отклонены с понятными сообщениями; валидный `mail.example.com` — обычный старт, все процессы supervisord поднимаются. `go vet`/`go test ./...` чистые.
-- **Решено, но ещё не реализовано:** пункт **C.4** плана. Герметичный контейнерный e2e отдельным Go-модулем `test/e2e/` поверх поставляемого compose, гейт перед публикацией образа по тегу, нативная матрица amd64/arm64 вместо qemu в `release.yml`; делается **после** B.1–B.3 (сделаны), стендовые проверки B.1/B.3 переезжают в него регрессиями. Замыкает очередь **D.5** — предрелизная проверка на уязвимости моделью Fable по всему дифу от `v1.0.0` плюс повторный проход по ТЗ 7.6; вместе с e2e это гейт перед тегом.
+- **C.4 реализован** (не выкачен на прод — это CI/тестовая инфраструктура, а не образ): герметичный контейнерный e2e отдельным Go-модулем `test/e2e/` (свой `go.mod`, не подхватывается `go test ./...` основного модуля) поверх поставляемого `deploy/docker-compose.yml` плюс `test/e2e/compose.override.yml` (самоподписанный сертификат, `PANEL_COOKIE_SECURE=false`, `SELFPOST_HOSTNAME=mail.e2e.test`, высокие порты `20465/20587/20080`, изолированный compose-проект `selfpost-e2e`, свой `--project-directory` — прод на том же хосте не задет). Герметичная почта: CoreDNS (`test/e2e/dns/Corefile` — авторитетна только для `e2e.test`, `file`-плагин с саб-директивой `reload` перечитывает `db.zone` по mtime, без сигналов) плюс `smtp-sink` из пакета postfix (`test/e2e/sink/`) как sink-MX. Сценарий (`test/e2e/*_test.go`): старт контейнера → все supervisord-программы `RUNNING` (`postfix-reload` — `STOPPED`) → токен из `/data/setup-token` → setup → login → добавление домена → DKIM-запись **скраплена со страницы панели** и опубликована в фейковую зону → добавление приложения → SMTP AUTH на 465 → письмо на sink → DKIM-подпись проверена (`go-msgauth/dkim` с кастомным `LookupTXT` через CoreDNS) против ключа **из DNS**, не из панели напрямую → send-log `queued → sent`. Негативы: без AUTH, relay на чужой домен без AUTH, sender/login mismatch (`reject_sender_login_mismatch` репортится Postfix'ом на RCPT, не MAIL — `smtpd_delay_reject=yes` по умолчанию), L1-лимит (anvil, override `RATE_LIMIT_MESSAGES_PER_IP=50` — специально высокий, чтобы остальные под-тесты не расходовали общий бюджет по IP раньше времени; сам тест шлёт до 60 раз, ждёт отказа), L2-лимит через панель (домен/приложение → `rejected`-строка в send-log), fail-open journal-milter'а (`supervisorctl stop panel`, письмо всё равно принято, контейнер жив), пустой/синтаксически неверный `SELFPOST_HOSTNAME` (отдельный один-разовый контейнер, не общий стенд), сессия переживает `docker restart` (плюс явное ожидание готовности smtps-порта после рестарта — панель и Postfix поднимаются независимо). `make e2e` — локальный/dev-server прогон. Найдено и исправлено по ходу стендовой проверки: `reload` — саб-директива `file`-плагина CoreDNS, а не отдельный топ-левел плагин (топ-левел `reload` следит за самим Corefile, не за зоной); `docker compose build.context` резолвится относительно `--project-directory`, а не относительно файла, где объявлен; `smtp-sink` отказывается стартовать от root без `-u`; `html/template` эскейпит `+` в `&#43;` даже в тексте — скрапер значений со страницы обязан `html.UnescapeString`; проверки состояния сразу после `up`/`restart` должны поллиться, а не разово опрашиваться (supervisord/postfix поднимаются не мгновенно). **Проверено на dev-сервере (`selfpost.example.com`)**: `make e2e` — зелёный (`go vet`/`gofmt -l` тоже чистые в обоих модулях). `release.yml` переработан: job `prepare` (версия из тега) → матрица `[ubuntu-latest, ubuntu-24.04-arm]` — каждая нативно собирает образ (`--load`), прогоняет e2e, пушит тег `X.Y.Z-amd64`/`X.Y.Z-arm64` → job `merge` — `docker buildx imagetools create` в единый тег `X.Y.Z`; `setup-qemu-action` убран. Не проверено вживую (нельзя без реального тега): сам workflow на GitHub Actions — синтаксис вычитан, логика идентична локальному `make e2e` пути.
+- **Дальше:** пункт **D.5** плана — предрелизная проверка на уязвимости моделью Fable по всему дифу от `v1.0.0` плюс повторный проход по ТЗ 7.6; вместе с e2e (C.4, готов) это гейт перед тегом релиза.
 - **Дальше — то, что перечислено в `implementation-plan.md`:** открытые вопросы закрыты, раздел E теперь только указатель на объём 2.x (входящий релей O1+ и роль администратора домена; 2FA снята с рассмотрения); остаются принятые риски безопасности (переехали в [security.md](security.md): `POST` без `Sec-Fetch-Site`/`Origin` пропускается, CSRF-токенов нет) и опциональная **Фаза O1+** (входящий релей, линия 2.x.x, требует согласования).
 - **Прод:** `selfpost.example.com`, реальный Let's Encrypt сертификат, живой e2e (DKIM/SPF pass). Контейнер там всё ещё на образе v1.0 — Фаза 14 в него не выкатывалась. При апгрейде: админа один раз разлогинит (сменилось имя cookie), а от reverse-proxy требуется передача исходного `Host` (Apache-фрагмент из `deploy/` это делает).
 
diff --git a/test/e2e/compose.override.yml b/test/e2e/compose.override.yml
new file mode 100644
index 0000000..e913561
--- /dev/null
+++ b/test/e2e/compose.override.yml
@@ -0,0 +1,82 @@
+
+# Hermetic e2e stand override for ../../deploy/docker-compose.yml (plan C.4).
+#
+# Applied as a second -f on top of the SHIPPED compose file, not a separate
+# test compose — the whole point is to exercise cap_drop/cap_add/
+# no-new-privileges exactly as documented, not a laxer stand-in. Run only via
+# `make e2e`, which also sets --project-directory to a scratch stage
+# directory so the base file's relative ./data and ./certs bind mounts land
+# there instead of next to a real deployment, and -p selfpost-e2e so the
+# containers/network never collide with a prod stack on the same host.
+#
+# `ports:` and `networks:` use the compose-spec `!override` merge tag to fully
+# replace the base file's list rather than concatenate with it (the default
+# merge behaviour for sequences) — otherwise the real 465/587/8080 would stay
+# published here too and collide with a prod container on the same host.
+
+services:
+  selfpost:
+    build:
+      # Resolved relative to --project-directory (.stage), NOT this file's own
+      # directory — compose build.context paths follow the project directory,
+      # unlike bind-mount volume paths, which is the same reason ./data and
+      # ./certs above need no path here at all.
+      context: ../../..
+      dockerfile: build/Dockerfile
+      args:
+        VERSION: e2e
+    image: selfpost:e2e
+    environment:
+      SELFPOST_HOSTNAME: mail.e2e.test
+      # Plain HTTP panel for the test client — no reverse proxy in this stand.
+      PANEL_COOKIE_SECURE: "false"
+      # High enough that the whole suite's other ~10 sends (positive path,
+      # level-2 test, the various rejected attempts) never trip it by
+      # accident — only the dedicated level-1 test (negative check 4, which
+      # loops well past this count) is meant to hit it.
+      RATE_LIMIT_MESSAGES_PER_IP: "50"
+      RATE_LIMIT_WINDOW_SECONDS: "60"
+    ports: !override
+      - "20465:465"
+      - "20587:587"
+      - "127.0.0.1:20080:8080"
+    dns:
+      - 10.77.0.10
+    networks: !override
+      e2e:
+    depends_on:
+      - coredns
+      - sink
+
+  # Fake authoritative DNS for e2e.test (plan C.4): serves the DKIM/host
+  # records the test harness publishes, so Postfix's outbound MX lookup for
+  # the sink and the panel's own dnscheck resolve inside the isolated network
+  # instead of hitting the real internet.
+  coredns:
+    image: coredns/coredns:latest
+    command: ["-conf", "/dns/Corefile"]
+    volumes:
+      - ./dns-stage:/dns
+    networks:
+      e2e:
+        ipv4_address: 10.77.0.10
+    ports:
+      - "20053:53/udp"
+
+  # Sink-MX: accepts any inbound mail and dumps each transaction to its own
+  # file under ./mail-stage, which the harness reads directly to check the
+  # DKIM-Signature header Postfix/OpenDKIM produced.
+  sink:
+    build:
+      context: ../sink
+    volumes:
+      - ./mail-stage:/mail
+    networks:
+      e2e:
+        ipv4_address: 10.77.0.11
+
+networks:
+  e2e:
+    ipam:
+      config:
+        - subnet: 10.77.0.0/24
diff --git a/test/e2e/dns/Corefile b/test/e2e/dns/Corefile
new file mode 100644
index 0000000..89c2eb5
--- /dev/null
+++ b/test/e2e/dns/Corefile
@@ -0,0 +1,16 @@
+# Authoritative fake zone for the e2e stand (plan C.4). Only e2e.test and its
+# subdomains are served — everything else is refused, which is fine: neither
+# the panel nor Postfix ever needs to resolve anything outside this zone here.
+#
+# The `file` plugin's own `reload` sub-directive (NOT the top-level `reload`
+# plugin, which only watches the Corefile itself) polls db.zone's mtime/serial
+# and reloads with no signal needed, so the test harness can rewrite the zone
+# file (to publish a domain's DKIM record) and just poll until the new record
+# is visible — no container restart, no sleep.
+e2e.test {
+    file /dns/db.zone {
+        reload 1s
+    }
+    log
+    errors
+}
diff --git a/test/e2e/dns_zone.go b/test/e2e/dns_zone.go
new file mode 100644
index 0000000..82a8420
--- /dev/null
+++ b/test/e2e/dns_zone.go
@@ -0,0 +1,120 @@
+package e2e
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// dnsAddr is where compose.override.yml publishes CoreDNS's UDP port, so the
+// test process (running on the host, not inside the e2e network) can resolve
+// through the same fake zone Postfix and the panel see via the `dns:` compose
+// directive pointing containers at CoreDNS's static 10.77.0.10.
+const dnsAddr = "127.0.0.1:20053"
+
+// e2eResolver returns a *net.Resolver that only ever talks to the fake zone's
+// CoreDNS, for both direct assertions in tests and as the DKIM verifier's
+// LookupTXT.
+func e2eResolver() *net.Resolver {
+	return &net.Resolver{
+		PreferGo: true,
+		Dial: func(ctx context.Context, network, _ string) (net.Conn, error) {
+			var d net.Dialer
+			return d.DialContext(ctx, network, dnsAddr)
+		},
+	}
+}
+
+// txtRecord is one name -> TXT value pair the harness publishes into the fake
+// zone (the DKIM record a domain's page told us to publish).
+type txtRecord struct {
+	name  string // relative to e2e.test, e.g. "selfpost._domainkey.sender"
+	value string
+}
+
+// writeZone (re)writes the whole authoritative db.zone for e2e.test: the fixed
+// records CoreDNS/sink need to be reachable, plus whatever TXT records the
+// test has published so far. CoreDNS's `reload 2s` (see dns/Corefile) picks up
+// the new mtime with no signal needed.
+func writeZone(stageDir string, extra []txtRecord) error {
+	var b strings.Builder
+	fmt.Fprintf(&b, "$ORIGIN e2e.test.\n$TTL 300\n")
+	fmt.Fprintf(&b, "@       IN SOA  ns.e2e.test. admin.e2e.test. ( %d 3600 900 604800 300 )\n", time.Now().Unix())
+	fmt.Fprintf(&b, "@       IN NS   ns.e2e.test.\n")
+	fmt.Fprintf(&b, "ns      IN A    10.77.0.10\n")
+	// The sink-MX: Postfix's outbound delivery for the recipient domain used by
+	// every positive-path send resolves here via the domain's implicit MX
+	// fallback to its own A record (RFC 5321) — no explicit MX record needed.
+	fmt.Fprintf(&b, "sink    IN A    10.77.0.11\n")
+	for _, r := range extra {
+		fmt.Fprintf(&b, "%s IN TXT %s\n", r.name, chunkTXT(r.value))
+	}
+	return os.WriteFile(filepath.Join(stageDir, "dns-stage", "db.zone"), []byte(b.String()), 0o644)
+}
+
+// chunkTXT splits a TXT value into <255-byte quoted strings (the DNS
+// <character-string> limit), which resolvers concatenate back into one value.
+// A DKIM RSA-2048 public key's base64 comfortably exceeds 255 bytes on its
+// own, so this is required, not defensive.
+func chunkTXT(value string) string {
+	const max = 255
+	var parts []string
+	for len(value) > max {
+		parts = append(parts, value[:max])
+		value = value[max:]
+	}
+	parts = append(parts, value)
+	var quoted []string
+	for _, p := range parts {
+		quoted = append(quoted, `"`+strings.ReplaceAll(p, `"`, `\"`)+`"`)
+	}
+	return strings.Join(quoted, " ")
+}
+
+// publishTXT rewrites the zone with rec added/replacing any prior record of
+// the same name, then blocks until CoreDNS actually serves the new value —
+// so the caller never has to sleep-and-hope for the reload interval to pass.
+func publishTXT(stageDir string, existing []txtRecord, rec txtRecord) ([]txtRecord, error) {
+	updated := make([]txtRecord, 0, len(existing)+1)
+	for _, r := range existing {
+		if r.name != rec.name {
+			updated = append(updated, r)
+		}
+	}
+	updated = append(updated, rec)
+	if err := writeZone(stageDir, updated); err != nil {
+		return nil, err
+	}
+	res := e2eResolver()
+	fqdn := rec.name + ".e2e.test"
+	err := waitFor(fmt.Sprintf("CoreDNS to serve TXT %s", fqdn), 20*time.Second, 300*time.Millisecond, func() (bool, error) {
+		ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+		defer cancel()
+		txt, err := res.LookupTXT(ctx, fqdn)
+		if err != nil {
+			return false, err
+		}
+		for _, t := range txt {
+			if t == rec.value {
+				return true, nil
+			}
+		}
+		return false, fmt.Errorf("got %v, want %q", txt, rec.value)
+	})
+	return updated, err
+}
+
+// lookupTXTFunc adapts e2eResolver to the shape dkim.VerifyOptions.LookupTXT
+// wants (a synchronous domain -> []string call, no context).
+func lookupTXTFunc() func(string) ([]string, error) {
+	res := e2eResolver()
+	return func(domain string) ([]string, error) {
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		return res.LookupTXT(ctx, domain)
+	}
+}
diff --git a/test/e2e/go.mod b/test/e2e/go.mod
new file mode 100644
index 0000000..e88d05d
--- /dev/null
+++ b/test/e2e/go.mod
@@ -0,0 +1,7 @@
+module codeberg.org/mix/selfpost/test/e2e
+
+go 1.26
+
+require github.com/emersion/go-msgauth v0.6.8
+
+require golang.org/x/crypto v0.15.0 // indirect
diff --git a/test/e2e/go.sum b/test/e2e/go.sum
new file mode 100644
index 0000000..2f7855e
--- /dev/null
+++ b/test/e2e/go.sum
@@ -0,0 +1,4 @@
+github.com/emersion/go-msgauth v0.6.8 h1:kW/0E9E8Zx5CdKsERC/WnAvnXvX7q9wTHia1OA4944A=
+github.com/emersion/go-msgauth v0.6.8/go.mod h1:YDwuyTCUHu9xxmAeVj0eW4INnwB6NNZoPdLerpSxRrc=
+golang.org/x/crypto v0.15.0 h1:frVn1TEaCEaZcn3Tmd7Y2b5KKPaZ+I32Q2OA3kYp5TA=
+golang.org/x/crypto v0.15.0/go.mod h1:4ChreQoLWfG3xLDer1WdlH5NdlQ3+mwnQq1YTKY+72g=
diff --git a/test/e2e/hostname_gate_test.go b/test/e2e/hostname_gate_test.go
new file mode 100644
index 0000000..f44b0e3
--- /dev/null
+++ b/test/e2e/hostname_gate_test.go
@@ -0,0 +1,101 @@
+package e2e
+
+import (
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+	"time"
+)
+
+// runEntrypoint runs the selfpost:e2e image's entrypoint standalone (no
+// compose, no DNS/sink sidecars — the SELFPOST_HOSTNAME gate in
+// build/entrypoint.sh runs and fails before anything else needs them) with
+// hostnameEnv substituted for SELFPOST_HOSTNAME, and returns its combined
+// output and whether it exited zero.
+func runEntrypoint(t *testing.T, hostnameEnv string) (output string, exitedZero bool) {
+	t.Helper()
+	dataDir := t.TempDir()
+
+	args := []string{
+		"run", "--rm",
+		"-e", "SELFPOST_HOSTNAME=" + hostnameEnv,
+		"-v", dataDir + ":/data",
+		"selfpost:e2e",
+	}
+	cmd := exec.Command("docker", args...)
+	cmd.Env = os.Environ()
+	out, err := cmd.CombinedOutput()
+	return string(out), err == nil
+}
+
+// TestHostnameGate is plan C.4 negative check 7: an empty or syntactically
+// invalid SELFPOST_HOSTNAME must fail the container fast with an explanatory
+// message (plan B.3), never a silent bad fallback. It runs the image directly
+// rather than through the shared stand — the whole point is to exercise
+// entrypoint.sh's gate before anything else in the container has a chance to
+// start, so it does not depend on TestE2E's stack at all.
+func TestHostnameGate(t *testing.T) {
+	cases := []struct {
+		name     string
+		hostname string
+		want     string
+	}{
+		{"empty", "", "SELFPOST_HOSTNAME is not set"},
+		{"scheme_and_port", "https://mail.example.com:465", "must be a bare hostname"},
+		{"bare_word_no_dot", "localhost", "fully-qualified domain name"},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			out, ok := runEntrypoint(t, c.hostname)
+			if ok {
+				t.Fatalf("container started with SELFPOST_HOSTNAME=%q, want a fatal exit\noutput:\n%s", c.hostname, out)
+			}
+			if !strings.Contains(out, c.want) {
+				t.Fatalf("SELFPOST_HOSTNAME=%q: output does not mention %q:\n%s", c.hostname, c.want, out)
+			}
+		})
+	}
+
+	t.Run("valid_hostname_starts", func(t *testing.T) {
+		out, ok := runEntrypointBackground(t, "mail.example.test")
+		if !ok {
+			t.Fatalf("container with a valid SELFPOST_HOSTNAME failed to start:\n%s", out)
+		}
+	})
+}
+
+// runEntrypointBackground starts the container detached and confirms
+// supervisord came up (rather than crash-looping) within a short timeout, then
+// removes it — the positive control for the gate cases above: a valid
+// hostname must not be caught by the same checks.
+func runEntrypointBackground(t *testing.T, hostnameEnv string) (output string, started bool) {
+	t.Helper()
+	dataDir := t.TempDir()
+	certDir := t.TempDir()
+	if err := writeSelfSignedCert(certDir+"/fullchain.pem", certDir+"/privkey.pem"); err != nil {
+		t.Fatalf("generate throwaway TLS cert: %v", err)
+	}
+	name := "selfpost-e2e-hostname-check"
+	_ = exec.Command("docker", "rm", "-f", name).Run()
+	defer exec.Command("docker", "rm", "-f", name).Run()
+
+	up := exec.Command("docker", "run", "-d", "--name", name,
+		"-e", "SELFPOST_HOSTNAME="+hostnameEnv,
+		"-v", dataDir+":/data",
+		"-v", certDir+":/etc/postfix/tls:ro",
+		"selfpost:e2e")
+	if out, err := up.CombinedOutput(); err != nil {
+		return string(out), false
+	}
+
+	err := waitFor("supervisord to report RUNNING processes", 20*time.Second, 300*time.Millisecond, func() (bool, error) {
+		out, err := exec.Command("docker", "exec", name, "supervisorctl", "-c", "/etc/supervisor/supervisord.conf", "status").CombinedOutput()
+		if strings.Contains(string(out), "RUNNING") {
+			return true, nil
+		}
+		return false, err
+	})
+	logs, _ := exec.Command("docker", "logs", name).CombinedOutput()
+	return string(logs), err == nil
+}
diff --git a/test/e2e/mail_helpers.go b/test/e2e/mail_helpers.go
new file mode 100644
index 0000000..2ac9ba8
--- /dev/null
+++ b/test/e2e/mail_helpers.go
@@ -0,0 +1,86 @@
+package e2e
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"time"
+
+	"github.com/emersion/go-msgauth/dkim"
+)
+
+// findSinkMessage polls the sink-MX's dump directory (see test/e2e/sink) for
+// a file whose contents contain token — the unique Subject-line marker every
+// test send carries — and returns it. Polling a directory listing rather than
+// sleeping a fixed duration is what keeps this deterministic even under a
+// slow CI runner (plan C.4).
+func findSinkMessage(stageDir, token string, timeout time.Duration) ([]byte, error) {
+	dir := filepath.Join(stageDir, "mail-stage")
+	var found []byte
+	err := waitFor(fmt.Sprintf("sink-MX to receive a message tagged %q", token), timeout, 300*time.Millisecond, func() (bool, error) {
+		entries, err := os.ReadDir(dir)
+		if err != nil {
+			return false, err
+		}
+		for _, e := range entries {
+			if e.IsDir() {
+				continue
+			}
+			b, err := os.ReadFile(filepath.Join(dir, e.Name()))
+			if err != nil {
+				continue
+			}
+			if bytes.Contains(b, []byte(token)) {
+				found = b
+				return true, nil
+			}
+		}
+		return false, fmt.Errorf("no dumped message contains %q yet (%d on disk)", token, len(entries))
+	})
+	return found, err
+}
+
+// verifyDKIM checks that raw (a message dumped by the sink) carries a DKIM
+// signature for domain that validates against the public key published in the
+// fake zone — i.e. exactly what a real receiver would check, using DNS the
+// harness itself controls instead of net.DefaultResolver (plan C.4).
+func verifyDKIM(raw []byte, domain string) error {
+	verifications, err := dkim.VerifyWithOptions(bytes.NewReader(raw), &dkim.VerifyOptions{
+		LookupTXT: lookupTXTFunc(),
+	})
+	if err != nil {
+		return fmt.Errorf("dkim verify: %w", err)
+	}
+	for _, v := range verifications {
+		if v.Domain == domain {
+			if v.Err != nil {
+				return fmt.Errorf("dkim signature for %s did not validate: %w", domain, v.Err)
+			}
+			return nil
+		}
+	}
+	return fmt.Errorf("no DKIM signature found for domain %s (got %d signature(s))", domain, len(verifications))
+}
+
+// connectFromPattern matches Postfix's smtpd "connect from ...[ADDR]" log
+// line, which is how the harness learns the source address Postfix itself
+// observed for a just-made connection (needed to configure a level-2 rate
+// limit's allowed-IPs list — the address a host-published port is seen as
+// inside the container depends on Docker's NAT and isn't worth hard-coding).
+var connectFromPattern = regexp.MustCompile(`connect from [^\[]*\[([0-9a-fA-F.:]+)\]`)
+
+// lastSMTPClientIP reads mail.log inside the selfpost container and returns
+// the most recent address Postfix's smtpd logged a connection from.
+func lastSMTPClientIP(s *stack) (string, error) {
+	out, err := s.execIn("selfpost", "tail", "-n", "200", "/var/log/mail.log")
+	if err != nil {
+		return "", err
+	}
+	matches := connectFromPattern.FindAllStringSubmatch(out, -1)
+	if len(matches) == 0 {
+		return "", fmt.Errorf("no \"connect from\" line in mail.log yet")
+	}
+	return matches[len(matches)-1][1], nil
+}
diff --git a/test/e2e/main_test.go b/test/e2e/main_test.go
new file mode 100644
index 0000000..a445a3a
--- /dev/null
+++ b/test/e2e/main_test.go
@@ -0,0 +1,236 @@
+// Package e2e is the hermetic container e2e gate (plan C.4): it drives the
+// shipped deploy/docker-compose.yml (plus a test-only override) exactly as an
+// administrator and their applications would, so the class of failure unit
+// tests cannot see — broken container wiring — has one place to be caught
+// before an image is published.
+//
+// It is a separate module on purpose (see ../../docs/implementation-plan.md,
+// item C.4): `go test ./...` in the main module never pulls this in, and its
+// test-only dependencies (DKIM verification) never enter the shipped
+// binaries' build graph.
+package e2e
+
+import (
+	"fmt"
+	"os"
+	"regexp"
+	"strings"
+	"testing"
+	"time"
+)
+
+var statusCellPattern = regexp.MustCompile(`<td>(queued|sent|deferred|bounced|rejected)</td>`)
+
+// h is the single shared stand for the whole ordered scenario in TestE2E.
+// TestHostnameGate does not use it — it spins its own disposable container.
+var h *stack
+
+func TestMain(m *testing.M) {
+	s, err := newStack()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "e2e: build stack:", err)
+		os.Exit(1)
+	}
+	h = s
+
+	if err := prepareStage(s); err != nil {
+		fmt.Fprintln(os.Stderr, "e2e: prepare stage:", err)
+		os.Exit(1)
+	}
+	if err := s.build(); err != nil {
+		fmt.Fprintln(os.Stderr, "e2e: build:", err)
+		os.Exit(1)
+	}
+	if err := s.up(); err != nil {
+		fmt.Fprintln(os.Stderr, "e2e: up:", err)
+		s.down()
+		os.Exit(1)
+	}
+
+	code := m.Run()
+
+	if code != 0 {
+		for _, svc := range []string{"selfpost", "coredns", "sink"} {
+			fmt.Fprintf(os.Stderr, "\n==== logs: %s ====\n%s\n", svc, s.logs(svc))
+		}
+	}
+	s.down()
+	os.Exit(code)
+}
+
+// senderDomain and recipient are fixed for the whole run: one sending domain
+// registered in the panel, and the sink-MX as the single recipient every
+// positive send targets.
+const (
+	senderDomain = "sender.e2e.test"
+	recipient    = "rcpt@sink.e2e.test"
+)
+
+// scenario carries state forward between the ordered subtests of TestE2E:
+// each later step needs something an earlier one produced (the domain id, the
+// DKIM record, the application credentials, an observed client IP).
+type scenario struct {
+	panel *panelClient
+
+	domainID        string
+	dkimName        string
+	dkimValue       string
+	dkimZoneRelName string // dkimName without the trailing ".e2e.test."
+	appLogin        string
+	appPassword     string
+	l2AppLogin      string
+	l2AppPassword   string
+	clientIP        string
+	zoneRecords     []txtRecord
+}
+
+func TestE2E(t *testing.T) {
+	sc := &scenario{}
+
+	t.Run("startup_processes_running", func(t *testing.T) {
+		if err := checkSupervisorProcesses(h); err != nil {
+			t.Fatal(err)
+		}
+	})
+
+	t.Run("setup_and_login", func(t *testing.T) {
+		token, err := readSetupToken(h.stageDir)
+		if err != nil {
+			t.Fatalf("read setup token: %v", err)
+		}
+		p, err := newPanelClient()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := p.setup(token, "e2eadmin", "correct-horse-battery-staple"); err != nil {
+			t.Fatalf("setup: %v", err)
+		}
+		if err := p.login("e2eadmin", "correct-horse-battery-staple"); err != nil {
+			t.Fatalf("login: %v", err)
+		}
+		sc.panel = p
+	})
+
+	t.Run("add_domain_and_publish_dkim", func(t *testing.T) {
+		id, err := sc.panel.addDomain(senderDomain)
+		if err != nil {
+			t.Fatalf("add domain: %v", err)
+		}
+		sc.domainID = id
+
+		name, value, err := sc.panel.dkimRecord(id)
+		if err != nil {
+			t.Fatalf("read dkim record: %v", err)
+		}
+		sc.dkimName, sc.dkimValue = name, value
+
+		relName, ok := trimZoneSuffix(name)
+		if !ok {
+			t.Fatalf("dkim record name %q is not under e2e.test", name)
+		}
+		sc.dkimZoneRelName = relName
+
+		records, err := publishTXT(h.stageDir, sc.zoneRecords, txtRecord{name: relName, value: value})
+		if err != nil {
+			t.Fatalf("publish dkim record to fake zone: %v", err)
+		}
+		sc.zoneRecords = records
+	})
+
+	t.Run("add_application", func(t *testing.T) {
+		login, password, err := sc.panel.addApplication(sc.domainID, "app1", "wildcard", "")
+		if err != nil {
+			t.Fatalf("add application: %v", err)
+		}
+		sc.appLogin, sc.appPassword = login, password
+	})
+
+	t.Run("send_verify_dkim_and_status", func(t *testing.T) {
+		token := uniqueToken("positive")
+		res := attemptSend(sendAttempt{
+			authLogin: sc.appLogin, authPassword: sc.appPassword,
+			from: "alerts@" + senderDomain, to: recipient,
+			subject: "e2e " + token, body: "hello from selfpost e2e",
+		})
+		if !res.ok() {
+			t.Fatalf("positive-path send failed: dial=%v auth=%v mail=%v rcpt=%v data=%v",
+				res.dialErr, res.authErr, res.mailErr, res.rcptErr, res.dataErr)
+		}
+
+		raw, err := findSinkMessage(h.stageDir, token, 30*time.Second)
+		if err != nil {
+			t.Fatalf("message never reached the sink: %v", err)
+		}
+		if err := verifyDKIM(raw, senderDomain); err != nil {
+			t.Fatalf("DKIM did not verify against the record the panel published: %v", err)
+		}
+
+		if err := waitFor("send-log row to reach status=sent", 30*time.Second, 500*time.Millisecond, func() (bool, error) {
+			rows, err := sc.panel.sendLogRows(senderDomain)
+			if err != nil {
+				return false, err
+			}
+			if containsCell(rows, "sent") {
+				return true, nil
+			}
+			return false, fmt.Errorf("send log not yet at status=sent (still %s)", firstStatusCell(rows))
+		}); err != nil {
+			t.Fatal(err)
+		}
+	})
+
+	t.Run("negative_level2_ratelimit_via_panel", func(t *testing.T) {
+		testLevel2RateLimit(t, sc)
+	})
+	t.Run("negative_sender_login_mismatch", func(t *testing.T) {
+		testSenderLoginMismatch(t, sc)
+	})
+	t.Run("negative_no_auth_rejected", func(t *testing.T) {
+		testNoAuthRejected(t, sc)
+	})
+	t.Run("negative_foreign_relay_rejected", func(t *testing.T) {
+		testForeignRelayRejected(t, sc)
+	})
+	t.Run("negative_journal_milter_fail_open", func(t *testing.T) {
+		testJournalMilterFailOpen(t, sc)
+	})
+	t.Run("session_survives_restart", func(t *testing.T) {
+		testSessionSurvivesRestart(t, sc)
+	})
+	t.Run("negative_level1_ratelimit", func(t *testing.T) {
+		testLevel1RateLimit(t, sc)
+	})
+}
+
+// trimZoneSuffix strips the "e2e.test" suffix a fully-qualified DKIM record
+// name carries, returning the name relative to the zone db.zone's $ORIGIN
+// expects (see dns_zone.go).
+func trimZoneSuffix(fqdn string) (string, bool) {
+	const suffix = ".e2e.test"
+	if len(fqdn) > len(suffix) && fqdn[len(fqdn)-len(suffix):] == suffix {
+		return fqdn[:len(fqdn)-len(suffix)], true
+	}
+	return "", false
+}
+
+var tokenCounter int
+
+// uniqueToken returns a short, human-readable, monotonically distinct marker
+// safe to embed in a Subject header and grep for in the sink's dump
+// directory.
+func uniqueToken(label string) string {
+	tokenCounter++
+	return fmt.Sprintf("%s-%d-%d", label, time.Now().UnixNano(), tokenCounter)
+}
+
+func containsCell(html, needle string) bool {
+	return strings.Contains(html, "<td>"+needle+"</td>")
+}
+
+func firstStatusCell(html string) string {
+	m := statusCellPattern.FindStringSubmatch(html)
+	if m == nil {
+		return "(no rows yet)"
+	}
+	return m[1]
+}
diff --git a/test/e2e/negative_test.go b/test/e2e/negative_test.go
new file mode 100644
index 0000000..c828be9
--- /dev/null
+++ b/test/e2e/negative_test.go
@@ -0,0 +1,224 @@
+package e2e
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+)
+
+// testLevel2RateLimit is plan C.4 negative check 5: a limit set through the
+// panel (not the environment) rejects the message that exceeds it, and the
+// rejection is visible in the send log — the panel -> DB -> milter path.
+func testLevel2RateLimit(t *testing.T, sc *scenario) {
+	ip, err := lastSMTPClientIP(h)
+	if err != nil {
+		t.Fatalf("determine observed SMTP client IP: %v", err)
+	}
+	sc.clientIP = ip
+
+	login, password, err := sc.panel.addApplication(sc.domainID, "l2app", "wildcard", "")
+	if err != nil {
+		t.Fatalf("add application: %v", err)
+	}
+	sc.l2AppLogin, sc.l2AppPassword = login, password
+
+	appID, err := sc.panel.applicationID(sc.domainID, login)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := sc.panel.setRateLimit(fmt.Sprintf("/applications/%s/ratelimit", appID), ip, 1, 3600); err != nil {
+		t.Fatalf("save application rate limit: %v", err)
+	}
+
+	from := "billing@" + senderDomain
+
+	first := attemptSend(sendAttempt{
+		authLogin: login, authPassword: password,
+		from: from, to: recipient, subject: "e2e " + uniqueToken("l2-first"), body: "ok",
+	})
+	if !first.ok() {
+		t.Fatalf("first message under the level-2 limit was rejected: %v", first.firstErr())
+	}
+
+	second := attemptSend(sendAttempt{
+		authLogin: login, authPassword: password,
+		from: from, to: recipient, subject: "e2e " + uniqueToken("l2-second"), body: "should be rejected",
+	})
+	if second.ok() {
+		t.Fatal("second message exceeding the level-2 limit was accepted, want rejected")
+	}
+	if second.mailErr == nil {
+		t.Fatalf("expected the level-2 rejection at MAIL FROM, got: dial=%v auth=%v rcpt=%v data=%v",
+			second.dialErr, second.authErr, second.rcptErr, second.dataErr)
+	}
+
+	if err := waitFor("a rejected row for l2app in the send log", 15*time.Second, 500*time.Millisecond, func() (bool, error) {
+		rows, err := sc.panel.sendLogRows(senderDomain)
+		if err != nil {
+			return false, err
+		}
+		if strings.Contains(rows, "l2app") && containsCell(rows, "rejected") {
+			return true, nil
+		}
+		return false, fmt.Errorf("no rejected row for l2app yet")
+	}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// testSenderLoginMismatch is plan C.4 negative check 2: an authenticated
+// application cannot send as a sender it does not own (spec 5.1 p.3,
+// reject_sender_login_mismatch) — the core anti-spoofing control.
+func testSenderLoginMismatch(t *testing.T, sc *scenario) {
+	res := attemptSend(sendAttempt{
+		authLogin: sc.appLogin, authPassword: sc.appPassword,
+		from: "someone@not-" + senderDomain, to: recipient,
+		subject: "e2e " + uniqueToken("mismatch"), body: "should be rejected",
+	})
+	if res.ok() {
+		t.Fatal("send with a sender the application does not own was accepted, want rejected")
+	}
+	// Postfix evaluates smtpd_sender_restrictions with smtpd_delay_reject=yes
+	// (the default): the mismatch is detected at MAIL FROM but the reject is
+	// only sent back at RCPT TO, so the error can land on either call here.
+	if res.mailErr == nil && res.rcptErr == nil {
+		t.Fatalf("expected the mismatch rejected at MAIL or RCPT, got: dial=%v auth=%v data=%v",
+			res.dialErr, res.authErr, res.dataErr)
+	}
+}
+
+// testNoAuthRejected is plan C.4 negative check 1: no SASL session, no mail.
+// The sender address used here belongs to no registered domain, so the
+// rejection is unambiguously about the missing AUTH and not sender ownership.
+func testNoAuthRejected(t *testing.T, sc *scenario) {
+	res := attemptSend(sendAttempt{
+		from: "anyone@unregistered.e2e.test", to: recipient,
+		subject: "e2e " + uniqueToken("noauth"), body: "should be rejected",
+	})
+	if res.ok() {
+		t.Fatal("unauthenticated send was accepted, want rejected")
+	}
+}
+
+// testForeignRelayRejected is plan C.4 negative check 3: a direct proof that
+// this is not an open relay — even the exact recipient the positive path just
+// delivered to is refused without AUTH (reject_unauth_destination).
+func testForeignRelayRejected(t *testing.T, sc *scenario) {
+	res := attemptSend(sendAttempt{
+		from: "anyone@unregistered.e2e.test", to: recipient,
+		subject: "e2e " + uniqueToken("relay"), body: "should be rejected",
+	})
+	if res.ok() {
+		t.Fatal("unauthenticated relay to an external destination was accepted, want reject_unauth_destination")
+	}
+	if res.rcptErr == nil && res.mailErr == nil {
+		t.Fatalf("expected rejection at MAIL or RCPT, got: dial=%v auth=%v data=%v", res.dialErr, res.authErr, res.dataErr)
+	}
+}
+
+// testJournalMilterFailOpen is plan C.4 negative check 6: the journal-milter
+// is monitoring-only and fails open (spec 7.3) — stopping the panel process
+// (which owns the milter socket) must not block mail, and must not crash the
+// container (crashexit only fires on PROCESS_STATE_FATAL, a clean supervisor
+// stop is STOPPED, see build/crashexit.py).
+func testJournalMilterFailOpen(t *testing.T, sc *scenario) {
+	if _, err := h.execIn("selfpost", "supervisorctl", "-c", "/etc/supervisor/supervisord.conf", "stop", "panel"); err != nil {
+		t.Fatalf("stop panel: %v", err)
+	}
+	t.Cleanup(func() {
+		_, _ = h.execIn("selfpost", "supervisorctl", "-c", "/etc/supervisor/supervisord.conf", "start", "panel")
+	})
+
+	res := attemptSend(sendAttempt{
+		authLogin: sc.appLogin, authPassword: sc.appPassword,
+		from: "alerts@" + senderDomain, to: recipient,
+		subject: "e2e " + uniqueToken("failopen"), body: "must still be accepted",
+	})
+	if !res.ok() {
+		t.Fatalf("send with the journal-milter down was rejected, want fail-open accept: dial=%v auth=%v mail=%v rcpt=%v data=%v",
+			res.dialErr, res.authErr, res.mailErr, res.rcptErr, res.dataErr)
+	}
+
+	if err := checkContainerAlive(h); err != nil {
+		t.Fatalf("container did not survive the panel stopping: %v", err)
+	}
+
+	// Restart the panel (also undone by t.Cleanup, redundantly and harmlessly,
+	// in case a later step needs it sooner than cleanup order guarantees).
+	if _, err := h.execIn("selfpost", "supervisorctl", "-c", "/etc/supervisor/supervisord.conf", "start", "panel"); err != nil {
+		t.Fatalf("restart panel: %v", err)
+	}
+	if err := waitFor("panel HTTP to answer again", 15*time.Second, 300*time.Millisecond, func() (bool, error) {
+		_, err := sc.panel.status()
+		return err == nil, err
+	}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// checkContainerAlive confirms the container is still up after a component
+// stop — the crashexit listener must not have brought it down.
+func checkContainerAlive(s *stack) error {
+	out, err := s.execIn("selfpost", "true")
+	if err != nil {
+		return fmt.Errorf("container not responding to exec: %v (%s)", err, out)
+	}
+	return nil
+}
+
+// testSessionSurvivesRestart is plan C.4 negative/regression check 8 (moved
+// here from the manual B.1 stand check, plan item C.4's closing note): the
+// login session (SQLite-backed, plan B.1) must survive `docker restart`.
+func testSessionSurvivesRestart(t *testing.T, sc *scenario) {
+	if err := h.restartSelfpost(); err != nil {
+		t.Fatalf("restart selfpost: %v", err)
+	}
+	if err := waitFor("panel HTTP to answer after restart", 30*time.Second, 500*time.Millisecond, func() (bool, error) {
+		_, err := sc.panel.status()
+		return err == nil, err
+	}); err != nil {
+		t.Fatal(err)
+	}
+	// The panel and Postfix come up independently after a restart (see
+	// waitForSMTPSReady) — the next subtest sends mail, so make sure smtpd is
+	// actually listening before this one returns.
+	if err := waitForSMTPSReady(30 * time.Second); err != nil {
+		t.Fatal(err)
+	}
+	resp, err := sc.panel.status()
+	if err != nil {
+		t.Fatalf("GET /status after restart: %v", err)
+	}
+	if resp.Request.URL.Path != "/status" {
+		t.Fatalf("session did not survive restart: landed on %s instead of /status", resp.Request.URL.Path)
+	}
+}
+
+// testLevel1RateLimit is plan C.4 negative check 4: the native Postfix anvil
+// backstop (smtpd_client_message_rate_limit, spec 5 p.5), set by the override
+// to RATE_LIMIT_MESSAGES_PER_IP=50, rejects once exceeded. It uses a dedicated
+// application with no level-2 limit of its own, and retries well past that
+// count, so the result is unambiguous regardless of how much of the shared
+// per-IP budget earlier subtests already spent (they stay well under 50
+// between them).
+func testLevel1RateLimit(t *testing.T, sc *scenario) {
+	login, password, err := sc.panel.addApplication(sc.domainID, "l1app", "wildcard", "")
+	if err != nil {
+		t.Fatalf("add application: %v", err)
+	}
+
+	const maxAttempts = 60
+	for i := 0; i < maxAttempts; i++ {
+		res := attemptSend(sendAttempt{
+			authLogin: login, authPassword: password,
+			from: "l1@" + senderDomain, to: recipient,
+			subject: "e2e " + uniqueToken("l1"), body: "rate limit probe",
+		})
+		if !res.ok() {
+			t.Logf("level-1 limit tripped on attempt %d/%d: %v", i+1, maxAttempts, res.firstErr())
+			return
+		}
+	}
+	t.Fatalf("level-1 rate limit (RATE_LIMIT_MESSAGES_PER_IP=5) never tripped after %d sends", maxAttempts)
+}
diff --git a/test/e2e/panel_client.go b/test/e2e/panel_client.go
new file mode 100644
index 0000000..8c38d29
--- /dev/null
+++ b/test/e2e/panel_client.go
@@ -0,0 +1,234 @@
+package e2e
+
+import (
+	"fmt"
+	"html"
+	"io"
+	"net/http"
+	"net/http/cookiejar"
+	"net/url"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"time"
+)
+
+// panelBaseURL is where compose.override.yml publishes the panel's plain-HTTP
+// port (PANEL_COOKIE_SECURE=false — no reverse proxy in this stand).
+const panelBaseURL = "http://127.0.0.1:20080"
+
+// panelClient drives the panel exactly as a browser would: an HTTP client
+// with a cookie jar, HTML forms posted as the templates render them, and
+// responses scraped for the bits a later step needs (a domain's id, an
+// application's one-shot password, the DKIM record to publish). This is
+// deliberate — it is also the only way to prove that what the panel tells an
+// administrator to publish is the record that actually verifies (plan C.4).
+type panelClient struct {
+	http *http.Client
+}
+
+func newPanelClient() (*panelClient, error) {
+	jar, err := cookiejar.New(nil)
+	if err != nil {
+		return nil, err
+	}
+	return &panelClient{http: &http.Client{
+		Jar:     jar,
+		Timeout: 15 * time.Second,
+	}}, nil
+}
+
+// readSetupToken reads the one-time setup URL SelfPost wrote to /data (bind
+// mounted at stageDir/data/setup-token) and returns just the token.
+func readSetupToken(stageDir string) (string, error) {
+	var raw []byte
+	err := waitFor("setup-token to appear", 30*time.Second, 300*time.Millisecond, func() (bool, error) {
+		b, err := os.ReadFile(filepath.Join(stageDir, "data", "setup-token"))
+		if err != nil {
+			return false, err
+		}
+		raw = b
+		return len(b) > 0, nil
+	})
+	if err != nil {
+		return "", err
+	}
+	u, err := url.Parse(strings.TrimSpace(string(raw)))
+	if err != nil {
+		return "", fmt.Errorf("parse setup token file: %w", err)
+	}
+	return strings.TrimPrefix(u.Path, "/setup/"), nil
+}
+
+func (c *panelClient) postForm(path string, form url.Values) (*http.Response, string, error) {
+	resp, err := c.http.PostForm(panelBaseURL+path, form)
+	if err != nil {
+		return nil, "", err
+	}
+	defer resp.Body.Close()
+	body, err := io.ReadAll(resp.Body)
+	return resp, string(body), err
+}
+
+func (c *panelClient) get(path string) (*http.Response, string, error) {
+	resp, err := c.http.Get(panelBaseURL + path)
+	if err != nil {
+		return nil, "", err
+	}
+	defer resp.Body.Close()
+	body, err := io.ReadAll(resp.Body)
+	return resp, string(body), err
+}
+
+// setup creates the administrator through the one-time /setup/<token> form.
+func (c *panelClient) setup(token, username, password string) error {
+	resp, body, err := c.postForm("/setup/"+token, url.Values{
+		"username":         {username},
+		"password":         {password},
+		"password_confirm": {password},
+	})
+	if err != nil {
+		return err
+	}
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("setup: status %d: %s", resp.StatusCode, body)
+	}
+	return nil
+}
+
+// login authenticates and stores the session cookie in the client's jar.
+func (c *panelClient) login(username, password string) error {
+	resp, body, err := c.postForm("/login", url.Values{
+		"username": {username},
+		"password": {password},
+	})
+	if err != nil {
+		return err
+	}
+	if resp.Request.URL.Path != "/status" {
+		return fmt.Errorf("login did not land on /status (landed on %s): %s", resp.Request.URL.Path, body)
+	}
+	return nil
+}
+
+// addDomain submits the add-domain form and returns its assigned id, read
+// back from the redirect target /domains/{id}.
+func (c *panelClient) addDomain(name string) (string, error) {
+	resp, body, err := c.postForm("/domains", url.Values{"name": {name}})
+	if err != nil {
+		return "", err
+	}
+	m := regexp.MustCompile(`^/domains/(\d+)$`).FindStringSubmatch(resp.Request.URL.Path)
+	if m == nil {
+		return "", fmt.Errorf("add domain %q: unexpected landing page %s: %s", name, resp.Request.URL.Path, body)
+	}
+	return m[1], nil
+}
+
+// dkimRecord fetches a domain's page and scrapes the DKIM DNS record it tells
+// the administrator to publish.
+func (c *panelClient) dkimRecord(domainID string) (name, value string, err error) {
+	_, body, err := c.get("/domains/" + domainID)
+	if err != nil {
+		return "", "", err
+	}
+	name, ok := extractCodeRow(body, "Host / name")
+	if !ok {
+		return "", "", fmt.Errorf("could not find DKIM record name on domain page")
+	}
+	value, ok = extractCodeRow(body, "Value")
+	if !ok {
+		return "", "", fmt.Errorf("could not find DKIM record value on domain page")
+	}
+	return name, value, nil
+}
+
+// addApplication submits the add-application form and returns the one-shot
+// login/password the panel renders inline (spec 7.6.1 — never recoverable
+// later, so this is the only place to read it).
+func (c *panelClient) addApplication(domainID, login, mode, addresses string) (appLogin, password string, err error) {
+	resp, body, err := c.postForm("/domains/"+domainID+"/applications", url.Values{
+		"login":     {login},
+		"mode":      {mode},
+		"addresses": {addresses},
+	})
+	if err != nil {
+		return "", "", err
+	}
+	if resp.StatusCode != http.StatusCreated {
+		return "", "", fmt.Errorf("add application %q: status %d: %s", login, resp.StatusCode, body)
+	}
+	appLogin, ok := extractCodeRow(body, "Login")
+	if !ok {
+		return "", "", fmt.Errorf("could not find new application login in response")
+	}
+	password, ok = extractCodeRow(body, "Password")
+	if !ok {
+		return "", "", fmt.Errorf("could not find new application password in response")
+	}
+	return appLogin, password, nil
+}
+
+// setRateLimit saves a level-2 differentiated limit (spec 7.4) on either a
+// domain (/domains/{id}/ratelimit) or an application (/applications/{id}/ratelimit).
+func (c *panelClient) setRateLimit(path, allowedIP string, maxMessages, windowSeconds int) error {
+	resp, body, err := c.postForm(path, url.Values{
+		"allowed_ips":    {allowedIP},
+		"max_messages":   {fmt.Sprintf("%d", maxMessages)},
+		"window_seconds": {fmt.Sprintf("%d", windowSeconds)},
+	})
+	if err != nil {
+		return err
+	}
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("set rate limit: status %d: %s", resp.StatusCode, body)
+	}
+	return nil
+}
+
+// sendLogRows returns the raw /sendlog/rows HTML fragment, filtered to one
+// domain, for polling a row's status without parsing full HTML into structs.
+func (c *panelClient) sendLogRows(domain string) (string, error) {
+	_, body, err := c.get("/sendlog/rows?domain=" + url.QueryEscape(domain))
+	return body, err
+}
+
+// status fetches the authenticated landing page — used after a container
+// restart to confirm the session cookie is still accepted (plan C.4 check 8).
+func (c *panelClient) status() (*http.Response, error) {
+	resp, _, err := c.get("/status")
+	return resp, err
+}
+
+// applicationID scrapes an application's numeric id off its domain page row,
+// keyed by login — needed to build /applications/{id}/ratelimit, which the
+// add-application response (just the login/password) does not carry.
+func (c *panelClient) applicationID(domainID, login string) (string, error) {
+	_, body, err := c.get("/domains/" + domainID)
+	if err != nil {
+		return "", err
+	}
+	pattern := `(?s)<td class="code">` + regexp.QuoteMeta(login) + `</td>.*?/applications/(\d+)/mode`
+	m := regexp.MustCompile(pattern).FindStringSubmatch(body)
+	if m == nil {
+		return "", fmt.Errorf("could not find application id for login %q", login)
+	}
+	return m[1], nil
+}
+
+// extractCodeRow scrapes the value of a "<label>LABEL</label> ... <span
+// class=\"code\">VALUE</span>" pair from a rendered panel page (see
+// internal/web/templates/domain_detail.html). It is deliberately anchored to
+// the label text rather than position, so it survives unrelated template
+// reordering. html/template's escaper is conservative about which characters
+// it entity-encodes in text nodes — a DKIM value's base64 "+" comes back as
+// "&#43;" — so the match is HTML-unescaped before returning.
+func extractCodeRow(body, label string) (string, bool) {
+	pattern := `<label>` + regexp.QuoteMeta(label) + `</label>\s*<div class="code-row">\s*<span class="code">([^<]*)</span>`
+	m := regexp.MustCompile(pattern).FindStringSubmatch(body)
+	if m == nil {
+		return "", false
+	}
+	return html.UnescapeString(strings.TrimSpace(m[1])), true
+}
diff --git a/test/e2e/process_check.go b/test/e2e/process_check.go
new file mode 100644
index 0000000..4b8ff05
--- /dev/null
+++ b/test/e2e/process_check.go
@@ -0,0 +1,61 @@
+package e2e
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// wantRunning are the supervisord programs that must be RUNNING once the
+// container is up (build/supervisord.conf). postfix-reload is deliberately
+// excluded: autostart=false, its healthy resting state is STOPPED.
+var wantRunning = []string{"opendkim", "panel", "postfix", "cert-reload", "logrotate"}
+
+// checkSupervisorProcesses shells into the container directly (not through the
+// panel's /status page) so it works before an administrator account even
+// exists — this is the first thing the harness checks after `docker compose
+// up` (plan C.4). Programs take a moment to leave STARTING right after the
+// container starts, so this polls rather than checking once.
+func checkSupervisorProcesses(s *stack) error {
+	var lastErr error
+	err := waitFor("all supervised programs to reach their steady state", 20*time.Second, 500*time.Millisecond, func() (bool, error) {
+		out, execErr := s.execIn("selfpost", "supervisorctl", "-c", "/etc/supervisor/supervisord.conf", "status")
+		// supervisorctl exits non-zero when any program is not RUNNING, which
+		// is expected for postfix-reload — parse the output regardless of exit
+		// status.
+		states := parseSupervisorStatus(out)
+		if len(states) == 0 {
+			lastErr = fmt.Errorf("supervisorctl status produced nothing to parse: %v\n%s", execErr, out)
+			return false, lastErr
+		}
+		for _, name := range wantRunning {
+			state, ok := states[name]
+			if !ok {
+				lastErr = fmt.Errorf("program %q not reported by supervisorctl:\n%s", name, out)
+				return false, lastErr
+			}
+			if state != "RUNNING" {
+				lastErr = fmt.Errorf("program %q is %s, want RUNNING:\n%s", name, state, out)
+				return false, lastErr
+			}
+		}
+		if state, ok := states["postfix-reload"]; ok && state != "STOPPED" {
+			lastErr = fmt.Errorf("program postfix-reload is %s, want STOPPED (autostart=false):\n%s", state, out)
+			return false, lastErr
+		}
+		return true, nil
+	})
+	return err
+}
+
+func parseSupervisorStatus(out string) map[string]string {
+	states := make(map[string]string)
+	for _, line := range strings.Split(out, "\n") {
+		fields := strings.Fields(line)
+		if len(fields) < 2 {
+			continue
+		}
+		states[fields[0]] = fields[1]
+	}
+	return states
+}
diff --git a/test/e2e/sink/Dockerfile b/test/e2e/sink/Dockerfile
new file mode 100644
index 0000000..016f405
--- /dev/null
+++ b/test/e2e/sink/Dockerfile
@@ -0,0 +1,26 @@
+# SMTP sink-MX for the e2e stand (plan C.4): accepts any mail on 25 and dumps
+# each transaction to its own file under /mail, so the test harness can read
+# the raw received message (headers + body, including the DKIM-Signature
+# Postfix/OpenDKIM added) straight off disk. smtp-sink ships in the postfix
+# package itself — no new dependency.
+FROM debian:bookworm-slim
+
+ENV DEBIAN_FRONTEND=noninteractive
+RUN echo "postfix postfix/mailname string localhost" | debconf-set-selections \
+ && echo "postfix postfix/main_mailer_type string Internet Site" | debconf-set-selections \
+ && apt-get update \
+ && apt-get install -y --no-install-recommends postfix \
+ && rm -rf /var/lib/apt/lists/*
+
+RUN mkdir -p /mail
+WORKDIR /mail
+
+# -d: one file per transaction (a pseudo-random suffix is appended
+# automatically), so the harness just watches the directory for a new file
+# instead of parsing a shared multi-message dump. Binding :25 needs root, and
+# smtp-sink refuses to run as root without an explicit -u to switch to after
+# binding — "-u root" satisfies that check while staying root throughout,
+# which is fine here: throwaway container, isolated e2e network, never
+# exposed.
+ENTRYPOINT ["/usr/sbin/smtp-sink"]
+CMD ["-u", "root", "-d", "/mail/%Y%m%d%H%M%S.", "0.0.0.0:25", "100"]
diff --git a/test/e2e/smtp_client.go b/test/e2e/smtp_client.go
new file mode 100644
index 0000000..1fe805a
--- /dev/null
+++ b/test/e2e/smtp_client.go
@@ -0,0 +1,142 @@
+package e2e
+
+import (
+	"crypto/tls"
+	"fmt"
+	"net"
+	"net/smtp"
+	"time"
+)
+
+// smtpsAddr is where compose.override.yml publishes the primary implicit-TLS
+// submission port (465 in the shipped compose, remapped here to stay off the
+// dev server's real ports).
+const smtpsAddr = "127.0.0.1:20465"
+
+// sendAttempt is one SMTP transaction against the stand. An empty authLogin
+// means "connect without AUTH" (negative checks 1/3); from/to are full
+// mailbox addresses.
+type sendAttempt struct {
+	authLogin, authPassword string
+	from, to                string
+	subject, body           string
+}
+
+// sendResult records the error (if any) at each stage the transaction
+// reached. Only the fields up to and including the first failure are set —
+// the rest are nil because that stage was never attempted, not because it
+// succeeded silently.
+type sendResult struct {
+	dialErr error
+	authErr error
+	mailErr error
+	rcptErr error
+	dataErr error
+}
+
+// ok reports whether the message was fully accepted (DATA closed cleanly).
+func (r sendResult) ok() bool {
+	return r.dialErr == nil && r.authErr == nil && r.mailErr == nil && r.rcptErr == nil && r.dataErr == nil
+}
+
+// firstErr returns whichever stage failed first, or nil if none did.
+func (r sendResult) firstErr() error {
+	for _, e := range []error{r.dialErr, r.authErr, r.mailErr, r.rcptErr, r.dataErr} {
+		if e != nil {
+			return e
+		}
+	}
+	return nil
+}
+
+// attemptSend drives one SMTP transaction over the implicit-TLS port, exactly
+// as an application client library would: TLS connect, EHLO, optional AUTH
+// PLAIN, MAIL/RCPT/DATA. The certificate is the stand's throwaway self-signed
+// one (see stage.go), so verification is skipped — Postfix's
+// smtpd_tls_security_level is "may" here just like production, this is purely
+// about the test client trusting an unknown CA.
+func attemptSend(a sendAttempt) sendResult {
+	var res sendResult
+
+	dialer := &net.Dialer{Timeout: 10 * time.Second}
+	conn, err := tls.DialWithDialer(dialer, "tcp", smtpsAddr, &tls.Config{
+		InsecureSkipVerify: true,
+		ServerName:         selfpostHostname,
+	})
+	if err != nil {
+		res.dialErr = fmt.Errorf("dial %s: %w", smtpsAddr, err)
+		return res
+	}
+	c, err := smtp.NewClient(conn, selfpostHostname)
+	if err != nil {
+		res.dialErr = fmt.Errorf("smtp handshake: %w", err)
+		_ = conn.Close()
+		return res
+	}
+	defer c.Close()
+
+	if err := c.Hello("e2e-client.e2e.test"); err != nil {
+		res.dialErr = err
+		return res
+	}
+
+	if a.authLogin != "" {
+		auth := smtp.PlainAuth("", a.authLogin, a.authPassword, selfpostHostname)
+		if err := c.Auth(auth); err != nil {
+			res.authErr = err
+			_ = c.Quit()
+			return res
+		}
+	}
+
+	if err := c.Mail(a.from); err != nil {
+		res.mailErr = err
+		_ = c.Quit()
+		return res
+	}
+	if err := c.Rcpt(a.to); err != nil {
+		res.rcptErr = err
+		_ = c.Quit()
+		return res
+	}
+	w, err := c.Data()
+	if err != nil {
+		res.dataErr = err
+		_ = c.Quit()
+		return res
+	}
+	msg := buildMessage(a)
+	if _, err := w.Write([]byte(msg)); err != nil {
+		res.dataErr = err
+	} else if err := w.Close(); err != nil {
+		res.dataErr = err
+	}
+	_ = c.Quit()
+	return res
+}
+
+// waitForSMTPSReady polls the implicit-TLS port until a bare TCP connect
+// succeeds. Used after a container (re)start: the panel's HTTP port and
+// Postfix's smtpd come up independently (postfix-wrapper.sh additionally
+// waits on both milter sockets before starting Postfix at all), so a caller
+// that only waited for the panel could still race a not-yet-listening smtpd.
+func waitForSMTPSReady(timeout time.Duration) error {
+	return waitFor("smtps port to accept connections", timeout, 300*time.Millisecond, func() (bool, error) {
+		conn, err := net.DialTimeout("tcp", smtpsAddr, 2*time.Second)
+		if err != nil {
+			return false, err
+		}
+		_ = conn.Close()
+		return true, nil
+	})
+}
+
+// buildMessage renders a minimal, valid RFC 5322 message. subject is expected
+// to carry a unique token so the harness can find this exact message in the
+// sink-MX's dump directory afterwards.
+func buildMessage(a sendAttempt) string {
+	return fmt.Sprintf(
+		"From: %s\r\nTo: %s\r\nSubject: %s\r\nDate: %s\r\nMIME-Version: 1.0\r\nContent-Type: text/plain; charset=utf-8\r\n\r\n%s\r\n",
+		a.from, a.to, a.subject, time.Now().UTC().Format(time.RFC1123Z), a.body,
+	)
+}
diff --git a/test/e2e/stack.go b/test/e2e/stack.go
new file mode 100644
index 0000000..f39bb06
--- /dev/null
+++ b/test/e2e/stack.go
@@ -0,0 +1,113 @@
+package e2e
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// project isolates this stand's containers/network from anything else on the
+// host (in particular a prod SelfPost stack in an unrelated compose project —
+// plan C.4). stageDir holds everything a single e2e run writes: the generated
+// TLS cert, the /data bind mount, the DNS zone CoreDNS serves, and the mail
+// sink's dump directory. It is never reused across runs.
+const project = "selfpost-e2e"
+
+// stack drives the compose lifecycle. baseCompose is the shipped
+// deploy/docker-compose.yml — deliberately the real one, not a parallel test
+// compose, so cap_drop/cap_add/no-new-privileges are exercised as documented
+// rather than a laxer stand-in (plan C.4).
+type stack struct {
+	repoRoot        string
+	baseComposeFile string
+	overrideFile    string
+	stageDir        string
+}
+
+func newStack() (*stack, error) {
+	wd, err := os.Getwd() // test/e2e, where go.mod lives
+	if err != nil {
+		return nil, err
+	}
+	repoRoot, err := filepath.Abs(filepath.Join(wd, "..", ".."))
+	if err != nil {
+		return nil, err
+	}
+	stageDir, err := filepath.Abs(filepath.Join(wd, ".stage"))
+	if err != nil {
+		return nil, err
+	}
+	return &stack{
+		repoRoot:        repoRoot,
+		baseComposeFile: filepath.Join(repoRoot, "deploy", "docker-compose.yml"),
+		overrideFile:    filepath.Join(wd, "compose.override.yml"),
+		stageDir:        stageDir,
+	}, nil
+}
+
+// compose runs `docker compose <args...>` against this stand's two files and
+// project directory/name, streaming output to the test log on failure.
+func (s *stack) compose(args ...string) ([]byte, error) {
+	full := append([]string{
+		"compose",
+		"-p", project,
+		"-f", s.baseComposeFile,
+		"-f", s.overrideFile,
+		"--project-directory", s.stageDir,
+	}, args...)
+	cmd := exec.Command("docker", full...)
+	// deploy/docker-compose.yml interpolates ${SELFPOST_HOSTNAME:?...} at
+	// compose-file parse time, straight from the process environment — that
+	// happens before the override file's `environment:` mapping is even
+	// looked at (that mapping only reaches the container, not compose's own
+	// variable interpolation), so it has to be set here too.
+	cmd.Env = append(os.Environ(), "SELFPOST_HOSTNAME="+selfpostHostname)
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &out
+	err := cmd.Run()
+	if err != nil {
+		return out.Bytes(), fmt.Errorf("docker %v: %w\n%s", full, err, out.String())
+	}
+	return out.Bytes(), nil
+}
+
+// execIn runs a command inside the running service container via `docker
+// compose exec -T`, the same way the harness checks supervisord/mail.log
+// state without going through the panel (so those checks work even before an
+// administrator account exists).
+func (s *stack) execIn(service string, args ...string) (string, error) {
+	full := append([]string{"exec", "-T", service}, args...)
+	out, err := s.compose(full...)
+	return string(out), err
+}
+
+func (s *stack) build() error {
+	_, err := s.compose("build")
+	return err
+}
+
+func (s *stack) up() error {
+	_, err := s.compose("up", "-d", "--force-recreate")
+	return err
+}
+
+// restartSelfpost restarts just the selfpost container the way `docker
+// restart` would (plan C.4 check 8: a session must survive that), without
+// tearing down the DNS/sink sidecars or their state.
+func (s *stack) restartSelfpost() error {
+	_, err := s.compose("restart", "selfpost")
+	return err
+}
+
+func (s *stack) down() {
+	_, _ = s.compose("down", "-v", "--remove-orphans")
+}
+
+// logs returns a service's combined stdout/stderr, for failure diagnostics.
+func (s *stack) logs(service string) string {
+	out, _ := s.compose("logs", "--no-color", service)
+	return string(out)
+}
diff --git a/test/e2e/stage.go b/test/e2e/stage.go
new file mode 100644
index 0000000..c036a75
--- /dev/null
+++ b/test/e2e/stage.go
@@ -0,0 +1,86 @@
+package e2e
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/pem"
+	"fmt"
+	"math/big"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// selfpostHostname is SELFPOST_HOSTNAME for the whole e2e stand: the
+// certificate's CN/SAN, the panel's SASL realm and Postfix's myhostname/HELO
+// all have to agree on it (plan B.3), so it is defined once here.
+const selfpostHostname = "mail.e2e.test"
+
+// prepareStage (re)creates the scratch directory compose.override.yml mounts
+// everything from: /data, the TLS cert Postfix serves on 465, the DNS zone
+// CoreDNS is authoritative for, and the sink-MX's dump directory. Called once
+// per run before `docker compose up`, so every run starts from a clean slate.
+func prepareStage(s *stack) error {
+	if err := os.RemoveAll(s.stageDir); err != nil {
+		return fmt.Errorf("clean stage dir: %w", err)
+	}
+	dirs := []string{"data", "certs", "dns-stage", "mail-stage"}
+	for _, d := range dirs {
+		if err := os.MkdirAll(filepath.Join(s.stageDir, d), 0o755); err != nil {
+			return fmt.Errorf("mkdir %s: %w", d, err)
+		}
+	}
+	if err := writeSelfSignedCert(
+		filepath.Join(s.stageDir, "certs", "fullchain.pem"),
+		filepath.Join(s.stageDir, "certs", "privkey.pem"),
+	); err != nil {
+		return err
+	}
+	corefile, err := os.ReadFile(filepath.Join("dns", "Corefile"))
+	if err != nil {
+		return fmt.Errorf("read Corefile: %w", err)
+	}
+	if err := os.WriteFile(filepath.Join(s.stageDir, "dns-stage", "Corefile"), corefile, 0o644); err != nil {
+		return fmt.Errorf("write Corefile: %w", err)
+	}
+	return writeZone(s.stageDir, nil)
+}
+
+// writeSelfSignedCert generates a throwaway RSA key + self-signed certificate
+// for selfpostHostname, valid for a day — this stand never outlives that.
+// Postfix's smtpd_tls_security_level is "may" (opportunistic), not enforced,
+// so the e2e SMTP client simply skips verification of it (plan C.4: "test
+// dependencies... zero new dependencies" — no need for a real CA here).
+func writeSelfSignedCert(certPath, keyPath string) error {
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		return fmt.Errorf("generate TLS key: %w", err)
+	}
+	serial, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
+	if err != nil {
+		return err
+	}
+	tmpl := &x509.Certificate{
+		SerialNumber:          serial,
+		Subject:               pkix.Name{CommonName: selfpostHostname},
+		DNSNames:              []string{selfpostHostname},
+		NotBefore:             time.Now().Add(-time.Hour),
+		NotAfter:              time.Now().Add(24 * time.Hour),
+		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign,
+		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+		BasicConstraintsValid: true,
+		IsCA:                  true,
+	}
+	der, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, &key.PublicKey, key)
+	if err != nil {
+		return fmt.Errorf("create certificate: %w", err)
+	}
+	certPEM := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: der})
+	if err := os.WriteFile(certPath, certPEM, 0o644); err != nil {
+		return err
+	}
+	keyPEM := pem.EncodeToMemory(&pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(key)})
+	return os.WriteFile(keyPath, keyPEM, 0o600)
+}
diff --git a/test/e2e/wait.go b/test/e2e/wait.go
new file mode 100644
index 0000000..22c890e
--- /dev/null
+++ b/test/e2e/wait.go
@@ -0,0 +1,29 @@
+package e2e
+
+import (
+	"fmt"
+	"time"
+)
+
+// waitFor polls check every interval until it returns true or timeout elapses.
+// Every wait in this suite goes through here — no fixed sleeps standing in for
+// a readiness check (plan C.4): a passing check ends the wait immediately, and
+// a timeout fails with what was being waited for, not a bare "timed out".
+func waitFor(what string, timeout, interval time.Duration, check func() (bool, error)) error {
+	deadline := time.Now().Add(timeout)
+	var lastErr error
+	for {
+		ok, err := check()
+		if ok {
+			return nil
+		}
+		lastErr = err
+		if time.Now().After(deadline) {
+			if lastErr != nil {
+				return fmt.Errorf("timed out after %s waiting for %s: %w", timeout, what, lastErr)
+			}
+			return fmt.Errorf("timed out after %s waiting for %s", timeout, what)
+		}
+		time.Sleep(interval)
+	}
+}