inbound journal: one row per recipient, and every row counts to the cap (audit M3)

Security audit 2026-10-17, finding M3, stage 1. inboundSession.RcptTo
appended every RCPT (the outbound session de-duplicates), and the end of
the message inserted a row per recipient one statement at a time inside
Postfix's callback; the cut to 10 000 rows every 500 counted only the log
tailer's rows. Postfix accepts a repeated recipient up to
smtpd_recipient_limit (1 000) and sends each to the milter, so one message
was a thousand rows, uncut until the six-hourly prune.

Now: the inbound milter folds a repeated recipient (case-insensitive, as
cleanup's been_here() with BH_FLAG_FOLD does, and as session.RcptTo does);
a message's rows go in one transaction (store.InsertInboundSpamMessage,
which replaces InsertInboundSpam in milter.InboundStore); the count and the
cut moved from the tailer into the store (countInboundWrites, an atomic
counter on Store) so the milter's rows, the tailer's and the tailer's
start-up re-reads all count. store.InboundSpamLogMaxRows is the one cap.

Tests: TestInboundRecipientRepeatedIsOneRow (1 000 RCPT in two cases, one
row); TestInboundJournalIsCutWhicheverWriterWrites (a small cap: a message's
rows and refusals together cross the mark, the table is cut); the tailer's
own counting test went with its counter. go test ./internal/store
./internal/milter ./internal/logtail (Windows-only Follow failures aside).
Also gofmt of a test line from the M2 commit.

Docs: security.md § Mail path, architecture.md; CHANGELOG; plan.

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

CHANGELOG.md

@@ -18,6 +18,11 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   milter before Postfix holds it to the login, so a client that never
   signed in, or another tenant's application, could use up a domain's
   limit. It is now the signed-in application's domain.
+- **One accepted message is one Inbound › Log row per recipient** (audit
+  M3): a recipient repeated in RCPT TO, up to Postfix's thousand, was a row
+  each, and the journal milter's rows did not count toward the 10 000-row
+  cut between the six-hourly prunes. Repeats are one row, a message's rows
+  are written in one transaction, and both writers count toward the cut.
 - **A stranger's recipient no longer writes an Inbound › Log row** (audit
   M2): Postfix quotes a refused recipient in its reply with the quotes
   removed, so `RCPT TO:<"; from=<x> to=<y> …"@domain>` spelled a second

docs/architecture.md

@@ -149,7 +149,8 @@ smtp/inet` removes Debian's stock smtpd). Outbound delivery still uses the
   start are written only if the same row is not there already. The
   journal is pruned every six hours: rows
   older than the `inbound_spam_log_retention_days` setting, then down to
-  10 000.
+  10 000 - and down to 10 000 every 500 rows either writer adds in between
+  (the store counts them, `countInboundWrites`).
   **Filter lists** (`inbound_spam_list`): allow and deny entries for the
   external filter - sender address, sender domain, client address or network -
   validated and normalised by `internal/inbound` (`lists.go`) and written as

docs/security-audit-2026-10-17.md

@@ -727,13 +727,16 @@ the argument.
   that an unauthenticated session reserves nothing. — **Opus**, 2 h.
   *Done 2026-10-17:* TestRateLimitIsNotReservedByAnotherSendersMailFrom;
   e2e `domain_limit_not_reserved_by_a_stranger`.
-- [ ] **M3.** De-duplicate recipients in `inboundSession.RcptTo` (case-folded,
+- [x] **M3.** De-duplicate recipients in `inboundSession.RcptTo` (case-folded,
   as the outbound session does); the milter's inserts count toward the prune
   trigger (a counter in the store beside `InsertInboundSpam`, the prune
   called from whichever side crosses 500); one transaction per message.
   Done when a unit test sends one message with the same recipient 1 000
   times and sees one row, and 600 milter rows with no refusal trigger the
-  cut. — **Opus**, 2 h.
+  cut. — **Opus**, 2 h. *Done 2026-10-17:* the counter and the cut moved
+  into the store (`countInboundWrites`, both writers),
+  `InsertInboundSpamMessage`; TestInboundRecipientRepeatedIsOneRow,
+  TestInboundJournalIsCutWhicheverWriterWrites.
 - [ ] **W1.** Refuse a custom report address on the hostname; exact hosted
   address only in `AllowedRecipients`; a tag required for every recipient on
   the hostname in `checkReportDomain`; the existing test inverted; the

docs/security.md

@@ -463,7 +463,13 @@ operator can act on it.
   `2.14.4` the reply ended at the *first* `; from=<`, which a stranger's
   recipient could supply - security audit 2026-10-17, M2.)
   Only a recipient at an inbound domain makes a row. The journal is cut to
-  10 000 rows every 500 rows the tailer writes, and by age every six hours.
+  10 000 rows every 500 rows written - the tailer's refusals and the journal
+  milter's accepted recipients together, counted in the store - and by age
+  every six hours. The journal milter writes a recipient given twice, in any
+  case, once, and a message's rows in one transaction. (Until `2.14.4` it
+  wrote every repeated RCPT and its rows were not counted toward the cut:
+  one message could be a thousand rows, uncut for six hours - security
+  audit 2026-10-17, M3.)
 - **Filter lists** reach another container as files: values are whitelisted on
   the way in (address, domain, canonical IP or network; no `/0`, no
   IPv4-mapped network), the writer refuses a stored value with whitespace, a

internal/logtail/inbound.go

@@ -146,15 +146,6 @@ const inboundDetailMax = 255
 // cap bounds what is left - messages the client abandoned - and a flood.
 const inboundQueueIDs = 4096
 
-// inboundPruneEvery is how many rows the tailer writes before it has the
-// journal cut back to its row cap, without waiting for the six-hourly prune.
-// Refusals are written at the rate strangers send RCPT TO. Postfix's one
-// limit on that, smtpd_client_recipient_rate_limit, is per client address and
-// off by default (postconf(5), Postfix 3.10.13) and not set here, so between
-// two prunes the journal would otherwise grow as fast as someone cared to
-// make it.
-const inboundPruneEvery = 500
-
 // inboundJournal is the per-run state of the inbound half.
 type inboundJournal struct {
 	st InboundStore
@@ -166,8 +157,6 @@ type inboundJournal struct {
 	port25 map[string]uint64
 	order  []queued
 	seq    uint64
-	// written counts the rows since the journal was last cut to its cap.
-	written int
 }
 
 type queued struct {
@@ -287,13 +276,9 @@ func (j *inboundJournal) handle(line string, now time.Time, catchingUp bool) {
 		log.Printf("log-tailer: inbound journal: record %s -> %s: %v", from, to, err)
 		return
 	}
-	if j.written++; j.written >= inboundPruneEvery {
-		j.written = 0
-		// The zero time removes nothing by age: this is the cap alone.
-		if _, err := j.st.PruneInboundSpam(time.Time{}, inboundSpamLogMaxRows); err != nil {
-			log.Printf("log-tailer: inbound journal: cut to %d rows: %v", inboundSpamLogMaxRows, err)
-		}
-	}
+	// Between the six-hourly prunes the store cuts the journal to its cap
+	// every so many rows, the journal milter's and these together
+	// (store.InboundSpamLogMaxRows).
 }
 
 func (j *inboundJournal) remember(queueID string) {
@@ -354,9 +339,8 @@ func lineTime(line string, now time.Time) (time.Time, string) {
 	return now, line
 }
 
-// The journal keeps at most this many rows whatever their age: refusals are
-// written at the rate strangers knock on port 25.
-const inboundSpamLogMaxRows = 10000
+// The journal keeps at most this many rows whatever their age.
+const inboundSpamLogMaxRows = store.InboundSpamLogMaxRows
 
 // inboundRetentionLoop prunes the inbound journal at once and then on the
 // cadence of the send log.

internal/logtail/inbound_test.go

@@ -305,7 +305,7 @@ func TestInboundHeloCannotWriteTheRow(t *testing.T) {
 func TestInboundRecipientCannotWriteTheRow(t *testing.T) {
 	forged := `; from=<ceo@example.net> to=<board@in.example.com> proto=ESMTP helo=<x>; `
 	for name, line := range map[string]string{
-		"recipient unknown": head + `postfix/inbound/smtpd[41]: NOQUEUE: reject: RCPT from x[203.0.113.9]: 550 5.1.1 <` + forged + `@in.example.com>: Recipient address rejected: User unknown in relay recipient table; from=<probe@example.net> to=<"` + forged + `"@in.example.com> proto=ESMTP helo=<audit.example>`,
+		"recipient unknown":               head + `postfix/inbound/smtpd[41]: NOQUEUE: reject: RCPT from x[203.0.113.9]: 550 5.1.1 <` + forged + `@in.example.com>: Recipient address rejected: User unknown in relay recipient table; from=<probe@example.net> to=<"` + forged + `"@in.example.com> proto=ESMTP helo=<audit.example>`,
 		"forged sender also in MAIL FROM": head + `postfix/inbound/smtpd[41]: NOQUEUE: reject: RCPT from x[203.0.113.9]: 550 5.1.1 <` + forged + `@in.example.com>: Recipient address rejected: User unknown in relay recipient table; from=<"x; from=<ceo@example.net"@a.example> to=<"` + forged + `"@in.example.com> proto=ESMTP helo=<audit.example>`,
 	} {
 		j, st := inboundFixture()
@@ -362,18 +362,6 @@ func TestInboundStaleArrivalDoesNotDropALiveID(t *testing.T) {
 	}
 }
 
-// Between the six-hourly prunes the tailer has the journal cut to its cap
-// itself, every so many rows: refusals come at the rate strangers send them.
-func TestInboundCutsTheJournalAsItWrites(t *testing.T) {
-	j, st := inboundFixture()
-	for i := 0; i < inboundPruneEvery*2+10; i++ {
-		j.handle(unknownRcp, tailNow, false)
-	}
-	if len(st.pruned) != 2 || st.pruned[0] != inboundSpamLogMaxRows {
-		t.Errorf("cuts = %v, want two, to %d rows", st.pruned, inboundSpamLogMaxRows)
-	}
-}
-
 // The queue ids waiting for a verdict are bounded.
 func TestInboundQueueIDsAreBounded(t *testing.T) {
 	j, _ := inboundFixture()

internal/milter/inbound.go

@@ -21,7 +21,7 @@ import (
 // and the list of inbound domains, since port 25 also takes mail that is not
 // the relay's. *store.Store satisfies it; tests substitute a fake.
 type InboundStore interface {
-	InsertInboundSpam(e store.InboundSpamEntry) error
+	InsertInboundSpamMessage(e store.InboundSpamEntry, rcpts []store.InboundRecipient) error
 	IsInboundDomain(name string) (bool, error)
 	ForgetInboundQueueID(queueID string) error
 }
@@ -83,8 +83,19 @@ func (s *inboundSession) MailFrom(from string, m *milter.Modifier) (milter.Respo
 // RcptTo keeps the recipient as Postfix logs it, so that the delivery lines
 // that carry the upstream's answer find the row (recipientAsLogged): a quoted
 // local part that needs no quotes is written without them.
+//
+// A recipient given twice - in any case - is one row, as on the outbound side
+// (session.RcptTo): Postfix accepts a repeated RCPT up to
+// smtpd_recipient_limit and sends the milter each, and one message used to be
+// a thousand rows (security audit 2026-10-17, M3).
 func (s *inboundSession) RcptTo(rcpt string, m *milter.Modifier) (milter.Response, error) {
-	s.rcpts = append(s.rcpts, recipientAsLogged(rcpt))
+	to := recipientAsLogged(rcpt)
+	for _, have := range s.rcpts {
+		if strings.EqualFold(have, to) {
+			return milter.RespContinue, nil
+		}
+	}
+	s.rcpts = append(s.rcpts, to)
 	return milter.RespContinue, nil
 }
 
@@ -215,6 +226,7 @@ func (s *inboundSession) record(queueID string) {
 			log.Printf("inbound-journal: detach older rows of %s: %v", queueID, err)
 		}
 	}
+	var rows []store.InboundRecipient
 	for _, to := range s.rcpts {
 		domain := domainOf(to)
 		ok, err := s.rec.IsInboundDomain(domain)
@@ -222,14 +234,13 @@ func (s *inboundSession) record(queueID string) {
 			log.Printf("inbound-journal: look up %s: %v", domain, err)
 			continue
 		}
-		if !ok {
-			continue
-		}
-		e.InboundDomain, e.To = domain, to
-		if err := s.rec.InsertInboundSpam(e); err != nil {
-			log.Printf("inbound-journal: record %s -> %s: %v", s.from, to, err)
+		if ok {
+			rows = append(rows, store.InboundRecipient{Domain: domain, To: to})
 		}
 	}
+	if err := s.rec.InsertInboundSpamMessage(e, rows); err != nil {
+		log.Printf("inbound-journal: record %s -> %d recipients: %v", s.from, len(rows), err)
+	}
 }
 
 // What the journal keeps of a verdict: enough to see why, on one line.

internal/milter/inbound_test.go

@@ -20,11 +20,14 @@ type fakeInbound struct {
 	forgot    []string
 }
 
-func (f *fakeInbound) InsertInboundSpam(e store.InboundSpamEntry) error {
+func (f *fakeInbound) InsertInboundSpamMessage(e store.InboundSpamEntry, rcpts []store.InboundRecipient) error {
 	if f.insertErr != nil {
 		return f.insertErr
 	}
-	f.rows = append(f.rows, e)
+	for _, r := range rcpts {
+		e.InboundDomain, e.To = r.Domain, r.To
+		f.rows = append(f.rows, e)
+	}
 	return nil
 }
 
@@ -400,3 +403,23 @@ func TestInboundRowsMeetTheirDeliveryLines(t *testing.T) {
 		t.Errorf("with no queue id: rows %+v, detached %q", rec.rows, rec.forgot)
 	}
 }
+
+// A recipient given again - in any case - is one row: Postfix accepts a
+// repeated RCPT up to smtpd_recipient_limit (1 000) and sends each, and one
+// message used to be a thousand rows (security audit 2026-10-17, M3).
+func TestInboundRecipientRepeatedIsOneRow(t *testing.T) {
+	rec := &fakeInbound{domains: map[string]bool{"in.example.com": true}}
+	s := inboundSessionFor(t, rec, false)
+	rcpts := make([]string, 0, 1000)
+	for i := 0; i < 1000; i++ {
+		if i%2 == 0 {
+			rcpts = append(rcpts, "listed@in.example.com")
+		} else {
+			rcpts = append(rcpts, "LISTED@in.example.com")
+		}
+	}
+	message(t, s, "sender@example.net", rcpts, nil)
+	if len(rec.rows) != 1 || rec.rows[0].To != "listed@in.example.com" {
+		t.Errorf("rows = %d (%+v), want one, spelt as first given", len(rec.rows), rec.rows)
+	}
+}

internal/store/inboundspam.go

@@ -63,9 +63,86 @@ type InboundSpamEntry struct {
 // inbound domain registered under the row's name now (audit 2026-10-12, L12).
 const inboundDomainID = `(SELECT d.id FROM inbound_domains d WHERE d.name = ?)`
 
-// InsertInboundSpam records one journal row. It is called from the inbound
-// journal milter's hot path and from the log tailer, so it returns any error
-// for the caller to log; neither of them may hold up mail over it.
+// InboundSpamLogMaxRows is how many rows the journal keeps whatever their
+// age: refusals are written at the rate strangers knock on port 25, and
+// accepted mail at the rate they send recipients.
+const InboundSpamLogMaxRows = 10000
+
+// inboundCapRows and inboundCapEvery are the cap and how many rows - from
+// either writer - are written before the journal is cut back to it, without
+// waiting for the six-hourly prune. Postfix's one limit on the rate of RCPT
+// TO, smtpd_client_recipient_rate_limit, is per client address and off by
+// default (postconf(5), Postfix 3.10.13) and not set here. Until 2.14.4 only
+// the log tailer counted its rows; the journal milter's - one per accepted
+// recipient - were cut only every six hours (security audit 2026-10-17, M3).
+// Variables so that a test can make them small.
+var (
+	inboundCapRows  = InboundSpamLogMaxRows
+	inboundCapEvery = int64(500)
+)
+
+// countInboundWrites adds n written rows to the count and, once it reaches
+// inboundCapEvery, cuts the journal to its cap. Two writers crossing the mark
+// together may both cut; the second finds nothing to do.
+func (s *Store) countInboundWrites(n int) error {
+	if n <= 0 || s.inboundWrites.Add(int64(n)) < inboundCapEvery {
+		return nil
+	}
+	s.inboundWrites.Store(0)
+	// The zero time removes nothing by age: this is the cap alone.
+	if _, err := s.PruneInboundSpam(time.Time{}, inboundCapRows); err != nil {
+		return fmt.Errorf("cut inbound_spam_log to %d rows: %w", inboundCapRows, err)
+	}
+	return nil
+}
+
+// InboundRecipient is one recipient of an accepted message at an inbound
+// domain: a row of InsertInboundSpamMessage.
+type InboundRecipient struct {
+	Domain string
+	To     string
+}
+
+// InsertInboundSpamMessage records the rows of one accepted message, one per
+// recipient, e with InboundDomain and To taken from each, in one transaction:
+// the journal milter writes inside Postfix's end-of-message callback on the
+// panel's one connection, and a thousand recipients are one commit, not a
+// thousand. It returns any error for the caller to log.
+func (s *Store) InsertInboundSpamMessage(e InboundSpamEntry, rcpts []InboundRecipient) error {
+	if len(rcpts) == 0 {
+		return nil
+	}
+	at := e.At
+	if at.IsZero() {
+		at = time.Now()
+	}
+	tx, err := s.db.Begin()
+	if err != nil {
+		return fmt.Errorf("insert inbound_spam_log: begin: %w", err)
+	}
+	defer tx.Rollback() //nolint:errcheck // a no-op after Commit
+	for _, r := range rcpts {
+		if _, err := tx.Exec(
+			`INSERT INTO inbound_spam_log
+				(inbound_domain, client_ip, from_addr, to_addr, subject, decision, engine, detail, created_at, inbound_domain_id,
+				 queue_id, upstream_status)
+			 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, `+inboundDomainID+`, ?, ?)`,
+			r.Domain, e.ClientIP, e.From, r.To, e.Subject, e.Decision, e.Engine, e.Detail,
+			at.UTC().Format(time.RFC3339), r.Domain, e.QueueID, e.Upstream,
+		); err != nil {
+			return fmt.Errorf("insert inbound_spam_log: %w", err)
+		}
+	}
+	if err := tx.Commit(); err != nil {
+		return fmt.Errorf("insert inbound_spam_log: commit: %w", err)
+	}
+	return s.countInboundWrites(len(rcpts))
+}
+
+// InsertInboundSpam records one journal row. It is called from the log
+// tailer (and was from the journal milter, which now writes a message's rows
+// at once), so it returns any error for the caller to log; it may not hold up
+// mail over it.
 func (s *Store) InsertInboundSpam(e InboundSpamEntry) error {
 	at := e.At
 	if at.IsZero() {
@@ -82,7 +159,7 @@ func (s *Store) InsertInboundSpam(e InboundSpamEntry) error {
 	if err != nil {
 		return fmt.Errorf("insert inbound_spam_log: %w", err)
 	}
-	return nil
+	return s.countInboundWrites(1)
 }
 
 // InsertInboundSpamOnce records a journal row unless the same row is there
@@ -109,7 +186,10 @@ func (s *Store) InsertInboundSpamOnce(e InboundSpamEntry) (bool, error) {
 		return false, fmt.Errorf("insert inbound_spam_log once: %w", err)
 	}
 	n, _ := res.RowsAffected()
-	return n > 0, nil
+	if n == 0 {
+		return false, nil
+	}
+	return true, s.countInboundWrites(1)
 }
 
 // updateUpstreamQuery puts one delivery attempt's result on the rows of a

internal/store/inboundspam_test.go

@@ -1,6 +1,7 @@
 package store
 
 import (
+	"fmt"
 	"strings"
 	"testing"
 	"time"
@@ -495,3 +496,59 @@ func TestInboundUpstreamHeldAndReusedQueueID(t *testing.T) {
 		t.Errorf("ForgetInboundQueueID does not use idx_inbound_spam_log_queue_id:\n%s", plan)
 	}
 }
+
+// The journal is cut to its cap every so many rows, whichever writer wrote
+// them: the journal milter's rows of accepted mail count as the tailer's
+// refusals do (security audit 2026-10-17, M3). One message's rows are one
+// call. Here the cap is 5 rows and the cut every 4.
+func TestInboundJournalIsCutWhicheverWriterWrites(t *testing.T) {
+	st := openTestStore(t)
+	rowsBefore, everyBefore := inboundCapRows, inboundCapEvery
+	inboundCapRows, inboundCapEvery = 5, 4
+	t.Cleanup(func() { inboundCapRows, inboundCapEvery = rowsBefore, everyBefore })
+	if _, err := st.AddInboundDomain("in.example.com"); err != nil {
+		t.Fatal(err)
+	}
+	count := func() int {
+		var n int
+		if err := st.db.QueryRow("SELECT COUNT(*) FROM inbound_spam_log").Scan(&n); err != nil {
+			t.Fatal(err)
+		}
+		return n
+	}
+	accepted := InboundSpamEntry{ClientIP: "203.0.113.7", From: "a@example.net", Decision: SpamDecisionAccept, Engine: SpamEngineSelfPost}
+	var rcpts []InboundRecipient
+	for i := 0; i < 3; i++ {
+		rcpts = append(rcpts, InboundRecipient{Domain: "in.example.com", To: fmt.Sprintf("r%d@in.example.com", i)})
+	}
+	// Three rows of one message, then two refusals: the fourth row crosses
+	// the mark, and the journal is cut to five.
+	if err := st.InsertInboundSpamMessage(accepted, rcpts); err != nil {
+		t.Fatal(err)
+	}
+	for i := 0; i < 2; i++ {
+		if err := st.InsertInboundSpam(InboundSpamEntry{InboundDomain: "in.example.com", ClientIP: "203.0.113.9", To: "x@in.example.com",
+			Decision: SpamDecisionReject, Engine: SpamEnginePostfix}); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if n := count(); n != 5 {
+		t.Fatalf("after 5 rows, %d (no cut yet past the cap)", n)
+	}
+	// A message of six more crosses the mark again: cut to five.
+	rcpts = nil
+	for i := 0; i < 6; i++ {
+		rcpts = append(rcpts, InboundRecipient{Domain: "in.example.com", To: fmt.Sprintf("m%d@in.example.com", i)})
+	}
+	if err := st.InsertInboundSpamMessage(accepted, rcpts); err != nil {
+		t.Fatal(err)
+	}
+	if n := count(); n != 5 {
+		t.Errorf("after a message of six over the mark, %d rows, want the cap of 5", n)
+	}
+	// The message's rows carry the domain by id, like the tailer's.
+	var missing int
+	if err := st.db.QueryRow("SELECT COUNT(*) FROM inbound_spam_log WHERE inbound_domain_id IS NULL").Scan(&missing); err != nil || missing != 0 {
+		t.Errorf("%d rows without a domain id (%v)", missing, err)
+	}
+}

internal/store/store.go

@@ -13,6 +13,7 @@ import (
 	"io/fs"
 	"os"
 	"sort"
+	"sync/atomic"
 
 	_ "modernc.org/sqlite" // pure-Go SQLite driver (no cgo), keeps the static build
 )
@@ -23,6 +24,10 @@ var migrationsFS embed.FS
 // Store wraps the database connection pool.
 type Store struct {
 	db *sql.DB
+
+	// inboundWrites counts journal rows written since the journal was last
+	// cut to its row cap, by both of its writers (countInboundWrites).
+	inboundWrites atomic.Int64
 }
 
 // Open opens (creating if needed) the SQLite database at path, enables WAL and