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/dkim_from_test.go

@@ -0,0 +1,84 @@
+package e2e
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/emersion/go-msgauth/dkim"
+)
+
+// passedFromLines reads the From: lines the sender check passes, the file its
+// unit test reads too, with the application's domain put in.
+func passedFromLines(repoRoot, domain string) ([]string, error) {
+	raw, err := os.ReadFile(filepath.Join(repoRoot, "internal", "milter", "testdata", "from-lines-passed.txt"))
+	if err != nil {
+		return nil, err
+	}
+	var out []string
+	for _, line := range strings.Split(string(raw), "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		s, err := strconv.Unquote(line)
+		if err != nil {
+			return nil, fmt.Errorf("%s: %w", line, err)
+		}
+		out = append(out, strings.ReplaceAll(s, "{domain}", domain))
+	}
+	return out, nil
+}
+
+// testSignedForItsDomainOnly: the sender check and OpenDKIM read From: with
+// two parsers, and that they agree was argued, never run (audit 2026-10-12,
+// stage 2). Every From: line the check passes for an application is sent as
+// that application, and what arrives must be signed for its domain - with a
+// signature that validates - and for no other: a line the check reads as the
+// application's own and OpenDKIM as another sender's would be signed with
+// that sender's key.
+func testSignedForItsDomainOnly(t *testing.T, sc *scenario) {
+	lines, err := passedFromLines(h.repoRoot, senderDomain)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, line := range lines {
+		token := uniqueToken("dkimfrom")
+		res := attemptSend(sendAttempt{
+			authLogin: sc.appLogin, authPassword: sc.appPassword,
+			from: "alerts@" + senderDomain, headerFrom: line, to: recipient,
+			subject: "e2e " + token, body: "which key signs this",
+		})
+		if !res.ok() {
+			t.Errorf("From: %q was refused: %v", line, res.firstErr())
+			continue
+		}
+		raw, err := findSinkMessage(h.stageDir, token, 30*time.Second)
+		if err != nil {
+			t.Errorf("From: %q never reached the sink: %v", line, err)
+			continue
+		}
+		verifications, err := dkim.VerifyWithOptions(bytes.NewReader(raw), &dkim.VerifyOptions{LookupTXT: lookupTXTFunc()})
+		if err != nil {
+			t.Errorf("From: %q: dkim verify: %v", line, err)
+			continue
+		}
+		valid := false
+		for _, v := range verifications {
+			if v.Domain != senderDomain {
+				t.Errorf("From: %q was signed for %s", line, v.Domain)
+			}
+			if v.Domain == senderDomain && v.Err == nil {
+				valid = true
+			}
+		}
+		if !valid {
+			t.Errorf("From: %q has no valid signature for %s (%d signature(s))", line, senderDomain, len(verifications))
+		}
+	}
+}

test/e2e/main_test.go

@@ -310,6 +310,9 @@ func TestE2E(t *testing.T) {
 	run("sender_check_holds_the_from_line", func(t *testing.T) {
 		testSenderCheck(t, sc)
 	})
+	run("passed_from_lines_signed_for_their_domain_only", func(t *testing.T) {
+		testSignedForItsDomainOnly(t, sc)
+	})
 	run("panel_down_defers_mail", func(t *testing.T) {
 		testPanelDownDefersMail(t, sc)
 	})

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)