diff --git a/CHANGELOG.md b/CHANGELOG.md
index c933d14..87e4a36 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -44,6 +44,15 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   panel from a stopped one. Seen on the e2e stand with the panel frozen, and
   now counted, once per message (audit 2026-10-12, M13).
 
+- **Application logins are one name whatever their case.** Postfix compares
+  the SASL login with the sender map without regard to case, the panel did
+  not: `Ops` in one domain and `ops` in another were two applications to the
+  panel and one sender to Postfix, so a tenant could send with another
+  tenant's envelope sender. A new login is now stored in lower case, and
+  migration `0008` refuses two logins that differ only in case. A login
+  stored earlier, or imported from another instance, keeps its case, since
+  its client uses it (audit 2026-10-12, H4).
+
 ### Added
 
 - **The delivery log measures its own guesses** (migration `0007`). A table
diff --git a/docs/audit-2026-10-12.md b/docs/audit-2026-10-12.md
index a157f6d..a0b9af7 100644
--- a/docs/audit-2026-10-12.md
+++ b/docs/audit-2026-10-12.md
@@ -642,7 +642,7 @@ has already run is never edited — the next file is added; the published
 
 ### Stage 2 — the tenant boundary and the registry (H4, M7, M9, M10) → in `2.5.0` (migration `0008`)
 
-- [ ] **Logins lower-cased** at validation (owner, 2026-10-12: fold, do not
+- [x] **Logins lower-cased** at validation (owner, 2026-10-12: fold, do not
    refuse) and `applications.login COLLATE NOCASE` (migration `0008` rebuilds
    the unique index). Before the migration a check
    that no two existing logins differ only in case (there is one
diff --git a/docs/roadmap.md b/docs/roadmap.md
index 435b3e8..1e8548f 100644
--- a/docs/roadmap.md
+++ b/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`) | 14/61 | [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`) | 15/61 | [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
diff --git a/internal/app/service.go b/internal/app/service.go
index 81978df..f3b8bbe 100644
--- a/internal/app/service.go
+++ b/internal/app/service.go
@@ -55,6 +55,7 @@ func (s *Service) Get(id int64) (store.Application, error) {
 // the map rebuild fails, everything is rolled back so we never leave an
 // application the panel cannot fully account for.
 func (s *Service) Create(domainID int64, login, mode string, rawAddresses []string) (store.Application, string, error) {
+	login = foldLogin(login)
 	addresses, err := s.validateForDomain(domainID, login, mode, rawAddresses)
 	if err != nil {
 		return store.Application{}, "", err
@@ -313,7 +314,7 @@ func (s *Service) CreateWithSettings(domainID int64, login string, set Settings,
 	}
 	if err := s.applyAccess(a.ID, set, recalc); err != nil {
 		_ = s.ClearRateLimit(a.ID)
-		s.rollbackCreate(a.ID, login)
+		s.rollbackCreate(a.ID, a.Login) // the login as stored, folded
 		_ = s.Resync()
 		return store.Application{}, "", err
 	}
diff --git a/internal/app/service_test.go b/internal/app/service_test.go
index 7a5e69f..39aa4d7 100644
--- a/internal/app/service_test.go
+++ b/internal/app/service_test.go
@@ -113,6 +113,40 @@ func TestServiceCreateListValidatesDomain(t *testing.T) {
 	}
 }
 
+// A new login is stored in lower case, and the sender map names it so: to
+// Postfix "Ops" and "ops" are one login (audit 2026-10-12, H4). A second
+// application that differs only in case is refused, created or imported,
+// whichever domain it is for.
+func TestServiceCreateFoldsTheLogin(t *testing.T) {
+	svc, st, rec, maps := newServiceHarness(t)
+	a1 := addDomain(t, st, "a.example")
+	b := addDomain(t, st, "b.example")
+
+	a, _, err := svc.Create(a1.ID, "Ops", store.AddressModeWildcard, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if a.Login != "ops" {
+		t.Fatalf("login stored as %q, want ops", a.Login)
+	}
+	if _, ok := rec.set["ops"]; !ok {
+		t.Fatalf("SASL account written as %v, want ops", rec.set)
+	}
+	if len(maps.last) != 1 || maps.last[0].Login != "ops" {
+		t.Fatalf("map bindings = %+v, want the login in lower case", maps.last)
+	}
+
+	if _, _, err := svc.Create(b.ID, "OPS", store.AddressModeWildcard, nil); !errors.Is(err, store.ErrLoginExists) {
+		t.Fatalf("OPS in another domain = %v, want ErrLoginExists", err)
+	}
+	if err := svc.ImportApplication(b.ID, "oPs", store.AddressModeWildcard, nil, "imported-password"); !errors.Is(err, store.ErrLoginExists) {
+		t.Fatalf("an imported oPs = %v, want ErrLoginExists", err)
+	}
+	if _, ok := rec.set["OPS"]; ok {
+		t.Fatal("the refused OPS got a SASL account")
+	}
+}
+
 func TestServiceCreateDuplicateLogin(t *testing.T) {
 	svc, st, _, _ := newServiceHarness(t)
 	d := addDomain(t, st, "example.com")
diff --git a/internal/app/validate.go b/internal/app/validate.go
index 8c72090..1cb368a 100644
--- a/internal/app/validate.go
+++ b/internal/app/validate.go
@@ -24,6 +24,17 @@ func invalid(format string, args ...any) error {
 	return &ValidationError{msg: fmt.Sprintf(format, args...)}
 }
 
+// foldLogin is a new application's login as it is stored: in lower case
+// (audit 2026-10-12, H4; owner: fold, do not refuse). Postfix compares the
+// SASL login with the sender map's without regard to case, so two logins
+// that differ only in case would be one sender to Postfix and two
+// applications to the panel. The store refuses such a pair as well
+// (idx_applications_login_nocase); a login imported from another instance is
+// not folded, since its SASL account and its client use it as it is.
+func foldLogin(login string) string {
+	return strings.ToLower(login)
+}
+
 // validateLogin enforces a strict server-side whitelist for the SASL login
 // (security.md). It intentionally excludes '@': the login is stored in sasldb2,
 // where '@' separates the user from the realm, so allowing it would change the
diff --git a/internal/store/applications_test.go b/internal/store/applications_test.go
index 5091e0c..d934102 100644
--- a/internal/store/applications_test.go
+++ b/internal/store/applications_test.go
@@ -38,6 +38,20 @@ func TestAddApplicationWildcard(t *testing.T) {
 	}
 }
 
+// Logins are unique without regard to case, whatever the caller passed
+// (idx_applications_login_nocase, migration 0008).
+func TestAddApplicationRefusesALoginDifferingOnlyInCase(t *testing.T) {
+	st := openTestStore(t)
+	a := addTestDomain(t, st, "a.example")
+	b := addTestDomain(t, st, "b.example")
+	if _, err := st.AddApplication(a.ID, "Ops", AddressModeWildcard, nil); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := st.AddApplication(b.ID, "ops", AddressModeWildcard, nil); !errors.Is(err, ErrLoginExists) {
+		t.Fatalf("ops beside Ops = %v, want ErrLoginExists", err)
+	}
+}
+
 func TestAddApplicationListStoresAddresses(t *testing.T) {
 	st := openTestStore(t)
 	d := addTestDomain(t, st, "example.com")
diff --git a/internal/store/migrations/0008_tenant_boundary.sql b/internal/store/migrations/0008_tenant_boundary.sql
new file mode 100644
index 0000000..336988e
--- /dev/null
+++ b/internal/store/migrations/0008_tenant_boundary.sql
@@ -0,0 +1,11 @@
+-- The tenant boundary (audit 2026-10-12, stage 2).
+
+-- An application login is unique without regard to case. Postfix compares
+-- the SASL login with the sender map's without regard to case, so "Ops" and
+-- "ops" were two applications to the panel and one sender to Postfix: a
+-- tenant could take another's envelope sender (audit H4). New logins are
+-- folded to lower case by the panel; a login stored earlier keeps its case,
+-- since its SASL account and its client use it. The column's own UNIQUE
+-- (case-sensitive) stays: SQLite cannot change a column's collation without
+-- rebuilding the table, and this index is the stricter rule.
+CREATE UNIQUE INDEX idx_applications_login_nocase ON applications (login COLLATE NOCASE);
diff --git a/internal/store/schema_test.go b/internal/store/schema_test.go
index 820cb64..6d9cfd8 100644
--- a/internal/store/schema_test.go
+++ b/internal/store/schema_test.go
@@ -104,7 +104,7 @@ func TestBaselineSchema(t *testing.T) {
 	}
 	rows.Close()
 	wantIdx := []string{
-		"idx_dmarc_report_records_report", "idx_dmarc_reports_domain", "idx_dmarc_reports_received",
+		"idx_applications_login_nocase", "idx_dmarc_report_records_report", "idx_dmarc_reports_domain", "idx_dmarc_reports_received",
 		"idx_inbound_spam_log_created_at", "idx_inbound_spam_log_domain_created_at",
 		"idx_send_log_app_login_created_at", "idx_send_log_created_at", "idx_send_log_domain", "idx_send_log_lines_queue_id", "idx_send_log_queue_id", "idx_send_log_rejected",
 		"idx_sessions_expires_at",
@@ -115,8 +115,8 @@ func TestBaselineSchema(t *testing.T) {
 	}
 
 	var version, appID int64
-	if err := st.db.QueryRow("PRAGMA user_version").Scan(&version); err != nil || version != 7 {
-		t.Errorf("user_version = %d, %v; want 7", version, err)
+	if err := st.db.QueryRow("PRAGMA user_version").Scan(&version); err != nil || version != 8 {
+		t.Errorf("user_version = %d, %v; want 8", version, err)
 	}
 	if err := st.db.QueryRow("PRAGMA application_id").Scan(&appID); err != nil || appID != applicationID {
 		t.Errorf("application_id = %#x, %v; want %#x", appID, err, applicationID)
@@ -192,8 +192,8 @@ func TestOpenMigratesA20Database(t *testing.T) {
 	}
 	defer st.Close()
 	var version int
-	if err := st.db.QueryRow("PRAGMA user_version").Scan(&version); err != nil || version != 7 {
-		t.Errorf("user_version = %d, %v; want 7", version, err)
+	if err := st.db.QueryRow("PRAGMA user_version").Scan(&version); err != nil || version != 8 {
+		t.Errorf("user_version = %d, %v; want 8", version, err)
 	}
 	if ok, err := st.UserExists(); err != nil || !ok {
 		t.Errorf("the user of the 2.0 database: exists = %v, %v", ok, err)