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>

internal/store/sendlog.go

@@ -20,8 +20,8 @@ const (
 	// refusal. The journal-milter refuses at MAIL FROM with a 4xx when a level-2
 	// rate limit or the application's client-address list says no (Reason is
 	// ReasonLimit or ReasonClient; such a row has no queue-id). The sender check
-	// refuses a From: line that is not the application's with a 550 (ReasonFrom;
-	// its row has the queue-id). Every refused row is left out of the level-2
+	// refuses a From: or Sender: line that is not the application's with a
+	// 550 (ReasonFrom, ReasonSender; its row has the queue-id). Every refused row is left out of the level-2
 	// message count (it was never sent).
 	StatusRejected = "rejected"
 )
@@ -38,7 +38,8 @@ type SendLogEntry struct {
 	To       string
 	Subject  string
 	// Reason and HeaderFrom are set on refused rows only: why the message
-	// was refused (Reason*), and for ReasonFrom what its From: line said.
+	// was refused (Reason*), and for ReasonFrom and ReasonSender the address
+	// of the line refused (the column's name is from before Sender: was).
 	Reason     string
 	HeaderFrom string
 	// ClientIP is the client a refusal at MAIL FROM came from (InsertRejected