diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1920354..d9b37fe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,9 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 - **A domain's DMARC reports are deleted with it** (audit A5/W2): reports
   belong to a domain by name, and a domain deleted and added again showed
   its old reports to whoever it was assigned to then.
+- **The sender map is rebuilt one change at a time** (audit W3): two
+  application changes at once could leave the map without the newer one
+  until the next change.
 - **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
diff --git a/docs/security-audit-2026-10-17.md b/docs/security-audit-2026-10-17.md
index 19f5764..df5faf7 100644
--- a/docs/security-audit-2026-10-17.md
+++ b/docs/security-audit-2026-10-17.md
@@ -750,8 +750,8 @@ the argument.
   **Sonnet**, 0.5 h. *Done 2026-10-17:* in `store.DeleteDomain`, one
   transaction with the domain; TestDeletingADomainDeletesItsReports. (The `domain_id` form is better and 2 h; take it if
   stage 1 is split across two sittings.)
-- [ ] **W3.** A mutex around `Resync`'s read + write + reload. — **Sonnet**,
-  0.5 h.
+- [x] **W3.** A mutex around `Resync`'s read + write + reload. — **Sonnet**,
+  0.5 h. *Done 2026-10-17:* TestResyncRunsOneAtATime.
 
 ### Stage 2 — the key group (~2.5 h) → patch release, then the test server's `certs/`
 
diff --git a/internal/app/service.go b/internal/app/service.go
index 1a18000..538778a 100644
--- a/internal/app/service.go
+++ b/internal/app/service.go
@@ -6,6 +6,8 @@
 package app
 
 import (
+	"sync"
+
 	"github.com/mixeme/selfpost/internal/postfix"
 	"github.com/mixeme/selfpost/internal/store"
 )
@@ -25,6 +27,12 @@ type Service struct {
 	store *store.Store
 	sasl  *SASLDB
 	pf    SenderMaps
+
+	// resyncMu makes Resync's read of the bindings and its write of the map
+	// one step: two application changes at once could otherwise write the
+	// map in the order opposite to their reads, leaving the older set until
+	// the next change (security audit 2026-10-17, W3).
+	resyncMu sync.Mutex
 }
 
 // NewService builds the application service over the shared store, the sasldb2
@@ -227,6 +235,8 @@ func (s *Service) PurgeDomainSASL(domainID int64) error {
 // from the manual reload button; it doubles as recovery if the map ever drifts
 // from the database.
 func (s *Service) Resync() error {
+	s.resyncMu.Lock()
+	defer s.resyncMu.Unlock()
 	bindings, err := s.store.ListBindings()
 	if err != nil {
 		return err
diff --git a/internal/app/service_test.go b/internal/app/service_test.go
index fb69806..15a6ce8 100644
--- a/internal/app/service_test.go
+++ b/internal/app/service_test.go
@@ -3,7 +3,10 @@ package app
 import (
 	"errors"
 	"path/filepath"
+	"sync"
+	"sync/atomic"
 	"testing"
+	"time"
 
 	"github.com/mixeme/selfpost/internal/postfix"
 	"github.com/mixeme/selfpost/internal/store"
@@ -505,3 +508,44 @@ func TestSaveSettingsIsAllOrNothing(t *testing.T) {
 		t.Errorf("unknown application = %v, want ErrApplicationNotFound", err)
 	}
 }
+
+// overlapMaps counts rebuilds that run at once.
+type overlapMaps struct {
+	inside, most atomic.Int32
+}
+
+func (o *overlapMaps) RebuildSenderLoginMaps([]postfix.Binding) error {
+	n := o.inside.Add(1)
+	for {
+		m := o.most.Load()
+		if n <= m || o.most.CompareAndSwap(m, n) {
+			break
+		}
+	}
+	time.Sleep(5 * time.Millisecond)
+	o.inside.Add(-1)
+	return nil
+}
+
+// Resync's read of the bindings and its write of the map are one step: two
+// at once would write in the order opposite to their reads and leave the
+// older set in the map (security audit 2026-10-17, W3).
+func TestResyncRunsOneAtATime(t *testing.T) {
+	svc, _, _, _ := newServiceHarness(t)
+	maps := &overlapMaps{}
+	svc.pf = maps
+	var wg sync.WaitGroup
+	for i := 0; i < 8; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if err := svc.Resync(); err != nil {
+				t.Error(err)
+			}
+		}()
+	}
+	wg.Wait()
+	if got := maps.most.Load(); got != 1 {
+		t.Errorf("%d rebuilds ran at once, want 1", got)
+	}
+}