sender check: the reply names no other domain (L8); which key signs, tested (stage 2)

L8: a From: line naming another sender of the server was refused with
"it names another sender of this server, <domain>" - a confirmation, to
whoever tries names, that the domain is hosted here. The reason now stops
before the domain; the line as written stays in the reply (the client's own
text) and on the refused message's page (header_from). Unit test: the reason
of every refusal names none of the server's other domains; e2e: the reason
of each sender-check refusal does not contain other.e2e.test.

Which key OpenDKIM signs with (owner, 2026-10-13): the check and OpenDKIM
read From: with two parsers, and their agreement was argued, never run.
internal/milter/testdata/from-lines-passed.txt holds the lines the check
passes for an application - bare and bracketed, quoted names with a comma,
escaped quotes and an address inside, an extension, three encoded charsets,
two folded lines, comments (nested too), a stranger's address in the name -
as Go-quoted strings with {domain}. The unit test wants each passed; the new
e2e case passed_from_lines_signed_for_their_domain_only sends each as app1
and wants every DKIM signature at the sink to be for the sender domain,
with one valid. All fifteen were. "(the shop) a@domain" - a comment before
the address, valid RFC 5322 - is refused by the check ("it must be one
address"); strict, not a hole, so it is not in the file. Groups are refused
too.

Audit plan lines ticked (two) with what was found, CHANGELOG [Unreleased],
roadmap 21/62.
Evidence on the test server for this tree (with a618ffb): go vet clean, go
test ./... 23 packages ok, make e2e exit 0 - the new case passed in 4 s.

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

test/e2e/sender_check_test.go

@@ -83,6 +83,10 @@ func testSenderCheck(t *testing.T, sc *scenario) {
 		// DATA for the client; it is permanent and says what is wrong.
 		if e := fmt.Sprint(res.dataErr); res.dataErr == nil || !strings.HasPrefix(e, "550 ") || !strings.Contains(e, "5.7.1 From:") {
 			t.Errorf("%s: want 550 5.7.1 naming the From: line at the end of DATA, got %v", c.name, res.firstErr())
+		} else if reason := e[strings.LastIndex(e, " - ")+3:]; strings.Contains(reason, otherDomain) {
+			// What the client wrote is said back; that the other domain is
+			// a sender of this server is not (audit 2026-10-12, L8).
+			t.Errorf("%s: the reason %q names %s", c.name, reason, otherDomain)
 		}
 		if _, err := findSinkMessage(h.stageDir, token, 2*time.Second); err == nil {
 			t.Errorf("%s: the refused message reached the sink", c.name)