sender check: a Sender: line is held to the rule From: is held to

The owner's decision (2026-10-16) on the security review's F1: a Sender:
line - the sending agent's, shown beside From: as "on behalf of", and
signed since 2.8.0 - was neither held to the login nor, before, signed: an
application could write its own From: and Sender: ceo@bank.com under this
server's signature. Now, when a message has one, it must be one address
the application may send as, and at most one such line (RFC 5322 § 3.6);
From: is judged first.

The refusal is "550 5.7.1 Sender: <address> - <why>", a send-log row of
reason "sender" (store.ReasonSender), tagged "wrong Sender"; the message
page names the line. The log tailer recovers such a row from cleanup's line
as it does a From: refusal (fromRefusalRe reads either). Help text as in
the design commit d9d9af3; guide, architecture, security and CHANGELOG say
it, security.md keeps Resent-From: (signed, not judged) as an accepted
risk.

Tests: TestSenderLineHeldToTheSameRule (own address, a name, another
application's, a stranger's, two addresses, two lines, From: first), the
recovery of a Sender: refusal line, the tag, the message page; e2e
sender_check_holds_the_sender_line (own Sender: passes and is signed,
another domain's refused at DATA and logged).

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

docs/plans/panel-dkim-signing.md

@@ -209,8 +209,9 @@ libmilter1.0.1.
 *Done (2026-10-16):* help and Health went with stage 2 (design `005922b`);
 the reviews ran on stages 1-2 and their findings are fixed in `c2f63d2`,
 with the first rollout's incident (the queue's owner numbers); the
-documents in the commit after it. Open: holding `Sender:` to the login
-(security review F1, the owner's decision), and go-milter v0.4.1 keeping
+documents in the commit after it. `Sender:` is held to the login like
+`From:` (security review F1; the owner's decision, 2026-10-16). Open:
+go-milter v0.4.1 keeping
 every header in a map of its own per session (security.md, accepted risks)
 - a header section the signer refuses past 1 MiB is still held by the
 library, on 465/587 and on port 25 alike, as before 2.8.0.