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>

CHANGELOG.md

@@ -86,6 +86,13 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   registry); the rows of a deleted domain are the global role's alone. The
   inbound log follows the same rule (audit 2026-10-12, L12).
 
+- **A refusal no longer says which other domain sends from this server.**
+  A `From:` line that named another sender of the server was refused with
+  that sender's domain in the reply, which confirmed to whoever tried names
+  that it is hosted here. The reply now stops at "it names another sender of
+  this server"; the line as written stays in the reply and on the refused
+  message's page (audit 2026-10-12, L8).
+
 ### Added
 
 - **The delivery log measures its own guesses** (migration `0007`). A table

docs/audit-2026-10-12.md

@@ -690,7 +690,7 @@ has already run is never edited — the next file is added; the published
    about the name in DNS, and whoever holds the name now is its reader.
 - [ ] Docs: security.md § Roles (the guard) and the struck accepted risk, guide
    § Domain export, schema-migrations.md. — **Sonnet**
-- [ ] **Which key OpenDKIM signs with, seen and not reasoned** (owner,
+- [x] **Which key OpenDKIM signs with, seen and not reasoned** (owner,
    2026-10-13, from the Not verified list of from-header-binding). The check
    and OpenDKIM read `From:` with two parsers; that they agree was argued
    from the check's two rules, never run. An e2e case sends, as one
@@ -701,7 +701,12 @@ has already run is never edited — the next file is added; the published
    shared with the check's unit test, so a line added to one is run by the
    other. A disagreement found is fixed in the check before the cut, and
    security.md § Mail path says the claim is tested. - **Opus**
-- [ ] **The 550 reply names no other domain** (L8): `sender.go:334` stops at
+   *Done 2026-10-14:* fifteen lines (`internal/milter/testdata/from-lines-passed.txt`),
+   each signed for the application's domain alone, the signature valid. No
+   disagreement. Groups are refused by the check, so none is in the file;
+   a comment before the address - `(the shop) a@domain`, valid RFC 5322 - is
+   refused too ("it must be one address"): strict, not a hole, left as it is.
+- [x] **The 550 reply names no other domain** (L8): `sender.go:334` stops at
    "it names another sender of this server"; the refused message's page
    still shows the `From:` line as written (`header_from`), so the operator
    loses nothing. Test pins the reply text; e2e's sender-check negatives

docs/roadmap.md

@@ -42,7 +42,7 @@ in `git log` and [CHANGELOG.md](../CHANGELOG.md).
 | from-header-binding | Hold a message's `From:` header against the application that sends it | shipped `2.4.0` | 9/9 | [plans/from-header-binding.md](plans/from-header-binding.md) |
 | panel-redesign | Panel redesign (accepted mockups, design contract) | shipped `2.0.0` | 27/27 | [plans/panel-redesign.md](plans/panel-redesign.md) |
 | schema-squash | Squash SQLite migrations into a 2.x baseline | built in panel-redesign (stage 1) | — | [plans/panel-redesign.md](plans/panel-redesign.md) |
-| audit-2026-10-12 | Follow-ups of the 2026-10-12 full-tree code audit, seven stages | in progress (stage 0 shipped `2.4.2`; next stages 1+2 → `2.5.0`) | 19/62 | [audit-2026-10-12.md](audit-2026-10-12.md) |
+| audit-2026-10-12 | Follow-ups of the 2026-10-12 full-tree code audit, seven stages | in progress (stage 0 shipped `2.4.2`; next stages 1+2 → `2.5.0`) | 21/62 | [audit-2026-10-12.md](audit-2026-10-12.md) |
 
 **Recommended order** (not binding; owner, 2026-10-09: feature order follows
 development convenience): **panel-redesign** shipped as `2.0.0` (2026-10-11) and

internal/milter/sender.go

@@ -331,7 +331,11 @@ func (s *senderSession) judge() (shown, why string, err error) {
 			return shown, "", fmt.Errorf("signing table: %w", err)
 		}
 		if signs {
-			return printable(raw), "it names another sender of this server, " + d, nil
+			// Which one is not said: the reply would confirm to whoever
+			// tries names that another tenant sends from this server (audit
+			// 2026-10-12, L8). The line as written is in the reply and on
+			// the refused message's page.
+			return printable(raw), "it names another sender of this server", nil
 		}
 	}
 	// And one that needs to know nothing of OpenDKIM's parser: outside quoted

internal/milter/sender_test.go

@@ -3,6 +3,7 @@ package milter
 import (
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -144,6 +145,14 @@ func TestSenderCheck(t *testing.T) {
 			if !strings.HasPrefix(text, "y 550 5.7.1 From: ") || !strings.Contains(text, c.wantText) {
 				t.Fatalf("reply %q, want a 550 5.7.1 saying %q", text, c.wantText)
 			}
+			// The reason names no other domain of the server (audit L8); the
+			// From: line before it is what the client wrote itself.
+			reason := text[strings.LastIndex(text, " - ")+3:]
+			for _, other := range []string{"example.com", "mail.example.org", "example.org"} {
+				if strings.Contains(reason, other) {
+					t.Fatalf("the reason %q names %s", reason, other)
+				}
+			}
 			if len(rows) != 1 || rows[0].Reason != store.ReasonFrom || rows[0].To != "someone@elsewhere.example" || rows[0].AppLogin != c.login {
 				t.Fatalf("log rows: %+v", rows)
 			}
@@ -271,3 +280,37 @@ func TestSenderCheckRefusalWithNullSenderHasItsDomain(t *testing.T) {
 		}
 	}
 }
+
+// passedFromLines reads testdata/from-lines-passed.txt with {domain} put in.
+func passedFromLines(t *testing.T, domain string) []string {
+	t.Helper()
+	raw, err := os.ReadFile(filepath.Join("testdata", "from-lines-passed.txt"))
+	if err != nil {
+		t.Fatal(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 {
+			t.Fatalf("%s: %v", line, err)
+		}
+		out = append(out, strings.ReplaceAll(s, "{domain}", domain))
+	}
+	return out
+}
+
+// Every line of the shared file is passed for the domain's application; the
+// e2e then checks OpenDKIM signs each for that domain alone.
+func TestSenderCheckPassesTheSharedLines(t *testing.T) {
+	bind := NewBindings(writeMaps(t, senderMaps))
+	for _, line := range passedFromLines(t, "shop.example.org") {
+		resp, rows := offer(t, bind, "shop", "shop@shop.example.org", [2]string{"From", line})
+		if resp != milter.RespContinue || len(rows) != 0 {
+			t.Errorf("From: %q refused: %s", line, replyText(resp))
+		}
+	}
+}

internal/milter/testdata/from-lines-passed.txt

@@ -0,0 +1,22 @@
+# From: lines the sender check lets through for an application of {domain}
+# (audit 2026-10-12, stage 2: which key OpenDKIM signs with, seen and not
+# reasoned). One Go-quoted string per line; {domain} is the application's
+# domain. Two tests read this file: the check's unit test
+# (internal/milter/sender_test.go) wants each line passed, and the e2e
+# (test/e2e/dkim_from_test.go) sends each one and wants the message signed
+# for {domain} and nothing else. A line added here is run by both.
+"a@{domain}"
+"<a@{domain}>"
+"\"The Shop\" <Orders@{domain}>"
+"orders+48213@{domain}"
+"\"shop@{domain}\" <a@{domain}>"
+"\"Orders, returns\" <a@{domain}>"
+"\"The \\\"Shop\\\"\" <a@{domain}>"
+"\"someone@gmail.example\" <a@{domain}>"
+"=?utf-8?b?0JzQsNCz0LDQt9C40L0=?= <a@{domain}>"
+"=?windows-1251?B?zODj4Ofo7Q==?= <a@{domain}>"
+"=?koi8-r?Q?=ED=C1=C7=C1=DA=C9=CE?= <a@{domain}>"
+"\"A very long display name of the shop\"\r\n <a@{domain}>"
+"=?utf-8?b?0JzQsNCz0LDQt9C40L0=?=\r\n\t<a@{domain}>"
+"a@{domain} (the shop)"
+"a@{domain} (the (nested) shop)"

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)