panel-notifications: findings of the technical and security reviews

Neither review (read-only agents, Opus and Fable, over 972a9f8..400aa76) has
a Critical finding. Both found the same serious one, and more:

- The notification about refused mail was built on what a rejected row says
  of itself. Its domain is the one in MAIL FROM, which the client typed and
  Postfix has not yet held against the login when the milter refuses: an
  application over its limit could have had the panel write a message per
  made-up domain, to another tenant's users among others. And the same
  status is written when an application is refused for its client address,
  which is no limit. Refusals are now counted by the domain the login is
  registered under, only for logins that exist and only while a limit is set
  on the login or its domain; one subject per sending domain. They are read
  through a partial index of their own (migration 0005): the query walked
  the whole send log every minute.
- Account and user e-mails were checked with the DMARC report validator,
  which refuses Gmail and the like - the owner's own kind of address. A new
  validate.AccountEmail accepts them and refuses a local part beginning "-".
- "Said" now means sent: what arises while nobody can be told, or while
  Postfix does not take the message, is told at a later check. Its ending
  with nobody left to tell is simply over.
- Two DNS checks must be 45 minutes apart as well as two: the last check
  before a restart and the first after it were minutes apart.
- The notifier's own mail waiting in the queue, and its bounce coming back
  to postmaster@ this server, are not the queue's subject. A listing cut at
  its size limit or with a line too long to read is no listing; postqueue -j
  gets 30 seconds.
- The tables must name the selector the panel shows, not only have the line.
- An a/mx lookup that did not answer inside the SPF check made the record
  "not authorising"; it is "cannot tell" now, for the pages too.
- A ceiling of 60 messages an hour; a pass stops between domains when the
  process is asked to stop; sendmail gets a WaitDelay; days are rounded as a
  person counts; a failed read of the switch shows it on, not off.

Recorded as accepted in security.md, with a roadmap candidate
(from-header-binding): an application can have a message signed as the
server's own name - or, as always, as another registered domain - by writing
it into the From: header, which nothing holds against the login.

Left as found: a recipient whose copy failed while another's went out is not
tried again; a certificate that lives under 14 days is reported at each
renewal.

On the test server, for this tree: go vet clean, go test ./... 23 packages
ok, make e2e ok (326 s).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/roadmap.md

@@ -35,10 +35,11 @@ in `git log` and [CHANGELOG.md](../CHANGELOG.md).
 | template-data-typing | Typed structs for template data instead of `map[string]any` | candidate | — | — |
 | structured-logging | `log/slog` with levels and fields | candidate | — | — |
 | review-2026-08-followups | Remaining findings of the 2026-08-19 code review | candidate | — | — |
-| panel-notifications | E-mail notifications about important events | in progress | 8/10 | [plans/panel-notifications.md](plans/panel-notifications.md) |
+| panel-notifications | E-mail notifications about important events | in progress | 10/10 | [plans/panel-notifications.md](plans/panel-notifications.md) |
 | delivery-log-storage | Keep a message's `mail.log` lines with its send-log row | shipped `2.2.0` | 8/8 | [plans/delivery-log-storage.md](plans/delivery-log-storage.md) |
 | password-reset | Password reset by e-mail | candidate | — | — |
 | phone-desktop-mode | A phone browser in "desktop site" mode gets the phone navigation | candidate | — | — |
+| from-header-binding | Hold a message's `From:` header against the application that sends it | candidate | — | — |
 | panel-redesign | Panel redesign (accepted mockups, design contract) | shipped `2.0.0` | 27/27 | [plans/panel-redesign.md](plans/panel-redesign.md) |
 | schema-squash | Squash SQLite migrations into a 2.x baseline | built in panel-redesign (stage 1) | — | [plans/panel-redesign.md](plans/panel-redesign.md) |
 
@@ -428,6 +429,41 @@ has one coherent layout, checked on a phone in "desktop site" mode and in a
 
 ---
 
+## from-header-binding
+
+**Problem (security review of panel-notifications, 2026-10-12):** Postfix
+holds the envelope sender against the SASL login
+(`reject_sender_login_mismatch`), and OpenDKIM chooses the signing key by the
+`From:` header. Nothing holds the header against the login. An application
+of one sending domain can therefore write another registered domain - or,
+since 2.3.0, the server's own name - into `From:` and have the message signed
+with that name's key: mail that passes DMARC as a domain the application has
+no right to, or that looks like a notification of the panel.
+
+**Goal:** a message whose `From:` names a domain this server signs for is
+accepted only from an application of that domain.
+
+**Boundary:** the journal milter reads `From:` (and `Sender:`) and refuses at
+end-of-headers when the header's domain is a signing name other than the
+login's own. Mail whose `From:` is a domain this server does not sign for is
+not touched - it goes out unsigned today and would after. The journal milter
+is fail-open, so with it down the hole is as it is now; whether that is
+enough or the check belongs somewhere stricter is part of the item.
+
+**Done when:** the e2e stand shows an application refused for another
+tenant's `From:` and for the hostname's, and accepted for its own.
+
+**Dependencies / risks:** it refuses mail - an application that today sends
+with a `From:` of another of the operator's domains stops working until it
+is given an application of that domain; parsing `From:` differently from
+OpenDKIM would leave a gap, so the check has to be the cruder one (does the
+header mention `@<signing name>`).
+
+**Version:** `2.x` MINOR; `candidate`. Until then:
+[security.md](security.md) § Accepted risks.
+
+---
+
 ## panel-redesign
 
 **Goal:** a panel an operator opens a few times a year and still finds their