docs: add preflight installation-check page plan and roadmap entry

Captures the design for a /preflight page that verifies instance-level infrastructure (rDNS, TLS, ports, HELO banner, proxy headers, DKIM socket, test email). Status: candidate.

Co-authored-by: Cursor <cursoragent@cursor.com>

CHANGELOG.md

@@ -12,6 +12,10 @@ rate limit is correct for a non-default level-1 window. Schema migration
 
 ### Added
 
+- Roadmap item and plan for `/preflight` installation check page
+  ([docs/plans/preflight.md](docs/plans/preflight.md)) — instance-level
+  infrastructure verification (rDNS, TLS, ports, HELO banner, proxy headers,
+  DKIM socket, test email). Status: candidate.
 - SQLite migration `0010_send_log_app_login_index.sql` —
   `idx_send_log_app_login_created_at`. The journal-milter counts an
   application's messages on every `MAIL FROM`, and the domain page computes

docs/plans/preflight.md

@@ -0,0 +1,71 @@
+# Plan: preflight (installation check page)
+
+**Status:** candidate  
+**Date:** 2026-08-19  
+**Version:** `1.x` MINOR; `candidate` until explicitly agreed.
+
+---
+
+## Goal
+
+A dedicated `/preflight` page (global-admin only) that runs infrastructure-level
+checks and helps the administrator confirm the instance is fully configured and
+ready to deliver mail. Each failed check shows an actionable recommendation
+(what to do or where to look).
+
+## Scope
+
+**In:**
+
+- rDNS / PTR verification (reuse `dnscheck.Checker.checkServer`)
+- TLS certificate validity + hostname match (CN/SAN contains `SELFPOST_HOSTNAME`)
+- Port connectivity: 25, 465, and 587 (if `SUBMISSION_ENABLE`) — dial own
+  public IP from inside the container, expect a 220 banner
+- HELO/EHLO banner — connect to localhost:25, verify Postfix announces
+  `SELFPOST_HOSTNAME` (not `localhost` or a container ID)
+- Trusted proxy headers:
+  - If `TRUSTED_PROXY_CIDR` set: verify peer IP is in range and
+    `X-Forwarded-For` is present
+  - If `PANEL_COOKIE_SECURE=true`: verify `X-Forwarded-Proto: https`
+  - If `TRUSTED_PROXY_CIDR` empty: informational warning (panel exposed
+    directly)
+- DKIM signing — OpenDKIM milter socket reachable (reuse `health.CheckSocket`)
+- Send test email — admin enters recipient address, system sends via
+  localhost:25, shows queued/error result
+
+**Out:**
+
+- Per-domain DNS checks (SPF, DKIM TXT, DMARC) — those live on domain detail
+  pages.
+- Delivery tracking of test email (only confirms local MTA accepted).
+
+## Done when
+
+An administrator can open `/preflight`, see a traffic-light summary of all
+infrastructure prerequisites, read specific fix instructions for any failures,
+and send a test email to verify end-to-end flow.
+
+## Risks
+
+- Port-connectivity check from inside the container may not reflect external
+  reachability if NAT hairpin is unsupported (document limitation).
+- Test email requires at least one configured domain (or falls back to
+  `postmaster@SELFPOST_HOSTNAME`).
+
+## Implementation checklist
+
+Target version cut: TBD (MINOR). One commit per step; code only after roadmap
+status is **agreed**. See [development.md](../development.md) § Plan checklists.
+
+- [ ] Port-connectivity helper (`internal/health/port.go`) — dial own IP, read
+      SMTP banner — **Sonnet**
+- [ ] HELO banner check (connect localhost:25, parse 220 line) — **Sonnet**
+- [ ] Hostname-in-certificate check (extend or wrap `health.CheckCertificate`)
+      — **Sonnet**
+- [ ] Handler `handlers_preflight.go` (assemble all checks, actionable
+      recommendations) — **Sonnet**
+- [ ] Test-email handler (`POST /preflight/test-email`, `net/smtp` to
+      localhost:25) — **Sonnet**
+- [ ] Template `preflight.html` (traffic-light cards + test-email form) —
+      **Sonnet**
+- [ ] Routes in `web.go`, nav link — **Sonnet**

docs/roadmap.md

@@ -28,6 +28,7 @@ in `git log` and [CHANGELOG.md](../CHANGELOG.md).
 
 | ID | Topic | Status | Progress | Plan |
 |---|---|---|---|---|
+| preflight | Installation check page (`/preflight`) | candidate | — | [plans/preflight.md](plans/preflight.md) |
 | contributing | `CONTRIBUTING.md` | candidate | — | — |
 | inbound-antispam-panel | Inbound antispam journal + allow/deny lists | agreed | 1/12 | [plans/inbound-antispam-panel.md](plans/inbound-antispam-panel.md) |
 | inbound-quarantine | Inbound spam quarantine (hold / review / release) | candidate | — | [plans/inbound-quarantine.md](plans/inbound-quarantine.md) |
@@ -137,6 +138,30 @@ yet.
 
 ---
 
+## preflight
+
+**Goal:** a `/preflight` page (global-admin only) that verifies instance-level
+infrastructure: rDNS/PTR, TLS certificate + hostname match, port 25/465/587
+reachability, HELO banner correctness, reverse-proxy headers, DKIM milter
+socket, and a test-email sender. Each check shows an actionable fix
+recommendation on failure.
+
+**Boundary:** instance infrastructure only; per-domain DNS checks remain on
+domain detail pages. Does not track test-email delivery beyond local MTA
+acceptance.
+
+**Done when:** the page loads, all checks run with traffic-light results and
+recommendations, and the test-email form successfully submits a message through
+Postfix.
+
+**Dependencies / risks:** port-connectivity check may not reflect external
+reachability behind NAT without hairpin; test email needs at least one domain or
+falls back to `postmaster@SELFPOST_HOSTNAME`.
+
+**Version:** `1.x` MINOR; `candidate`.
+
+---
+
 ## schema-squash
 
 **Goal:** when 2.x is cut, stop shipping the 1.x migration chain in the binary