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>

test/e2e/main_test.go

@@ -327,6 +327,9 @@ func TestE2E(t *testing.T) {
 	run("sender_check_holds_the_from_line", func(t *testing.T) {
 		testSenderCheck(t, sc)
 	})
+	run("sender_check_holds_the_sender_line", func(t *testing.T) {
+		testSenderLine(t, sc)
+	})
 	run("passed_from_lines_signed_for_their_domain_only", func(t *testing.T) {
 		testSignedForItsDomainOnly(t, sc)
 	})