diff --git a/ctl_test.go b/ctl_test.go
index 517e653..aaa01bb 100644
--- a/ctl_test.go
+++ b/ctl_test.go
@@ -327,7 +327,10 @@ func TestCtl(t *testing.T) {
 	testctl(func(ctl *ctl) {
 		acc, err := store.OpenAccount(ctl.log, "mjl")
 		tcheck(t, err, "open account")
-		defer acc.Close()
+		defer func() {
+			acc.Close()
+			acc.CheckClosed()
+		}()
 
 		content := []byte("Subject: hi\r\n\r\nbody\r\n")
 
diff --git a/imapserver/fuzz_test.go b/imapserver/fuzz_test.go
index fe57563..94ffced 100644
--- a/imapserver/fuzz_test.go
+++ b/imapserver/fuzz_test.go
@@ -69,7 +69,10 @@ func FuzzServer(f *testing.F) {
 	if err != nil {
 		f.Fatalf("open account: %v", err)
 	}
-	defer acc.Close()
+	defer func() {
+		acc.Close()
+		acc.CheckClosed()
+	}()
 	err = acc.SetPassword(log, password0)
 	if err != nil {
 		f.Fatalf("set password: %v", err)
diff --git a/imapserver/server_test.go b/imapserver/server_test.go
index 6fc290e..a0564b2 100644
--- a/imapserver/server_test.go
+++ b/imapserver/server_test.go
@@ -310,6 +310,7 @@ func (tc *testconn) close() {
 	}
 	err := tc.account.Close()
 	tc.check(err, "close account")
+	// no account.CheckClosed(), the tests open accounts multiple times.
 	tc.account = nil
 	tc.client.Close()
 	tc.serverConn.Close()
diff --git a/smtpserver/fuzz_test.go b/smtpserver/fuzz_test.go
index d276d72..ef5fc6b 100644
--- a/smtpserver/fuzz_test.go
+++ b/smtpserver/fuzz_test.go
@@ -41,7 +41,10 @@ func FuzzServer(f *testing.F) {
 	if err != nil {
 		f.Fatalf("open account: %v", err)
 	}
-	defer acc.Close()
+	defer func() {
+		acc.Close()
+		acc.CheckClosed()
+	}()
 	err = acc.SetPassword(log, "testtest")
 	if err != nil {
 		f.Fatalf("set password: %v", err)
diff --git a/smtpserver/server_test.go b/smtpserver/server_test.go
index ae3b33d..4aa6a3d 100644
--- a/smtpserver/server_test.go
+++ b/smtpserver/server_test.go
@@ -139,6 +139,7 @@ func (ts *testserver) close() {
 	ts.switchStop()
 	err := ts.acc.Close()
 	tcheck(ts.t, err, "closing account")
+	ts.acc.CheckClosed()
 	ts.acc = nil
 }
 
@@ -1367,7 +1368,10 @@ func TestCatchall(t *testing.T) {
 
 	acc, err := store.OpenAccount(pkglog, "catchall")
 	tcheck(t, err, "open account")
-	defer acc.Close()
+	defer func() {
+		acc.Close()
+		acc.CheckClosed()
+	}()
 	n, err = bstore.QueryDB[store.Message](ctxbg, acc.DB).Count()
 	tcheck(t, err, "checking delivered messages to catchall account")
 	tcompare(t, n, 1)
diff --git a/store/account_test.go b/store/account_test.go
index 77a8e64..732c6d4 100644
--- a/store/account_test.go
+++ b/store/account_test.go
@@ -38,6 +38,7 @@ func TestMailbox(t *testing.T) {
 	defer func() {
 		err = acc.Close()
 		tcheck(t, err, "closing account")
+		acc.CheckClosed()
 	}()
 	defer Switchboard()()
 
diff --git a/store/export_test.go b/store/export_test.go
index e2abad5..3afb3d1 100644
--- a/store/export_test.go
+++ b/store/export_test.go
@@ -24,7 +24,10 @@ func TestExport(t *testing.T) {
 	mox.MustLoadConfig(true, false)
 	acc, err := OpenAccount(pkglog, "mjl")
 	tcheck(t, err, "open account")
-	defer acc.Close()
+	defer func() {
+		acc.Close()
+		acc.CheckClosed()
+	}()
 	defer Switchboard()()
 
 	log := mlog.New("export", nil)
diff --git a/store/threads_test.go b/store/threads_test.go
index fa63e5d..a273f6f 100644
--- a/store/threads_test.go
+++ b/store/threads_test.go
@@ -24,6 +24,7 @@ func TestThreadingUpgrade(t *testing.T) {
 	defer func() {
 		err = acc.Close()
 		tcheck(t, err, "closing account")
+		acc.CheckClosed()
 	}()
 	defer Switchboard()()
 
@@ -116,6 +117,7 @@ func TestThreadingUpgrade(t *testing.T) {
 	dbpath := acc.DBPath
 	err = acc.Close()
 	tcheck(t, err, "close account")
+	acc.CheckClosed()
 
 	// Now clear the threading upgrade, and the threading fields and close the account.
 	// We open the database file directly, so we don't trigger the consistency checker.
diff --git a/webaccount/account_test.go b/webaccount/account_test.go
index f011bce..4d8a01a 100644
--- a/webaccount/account_test.go
+++ b/webaccount/account_test.go
@@ -97,6 +97,7 @@ func TestAccount(t *testing.T) {
 	defer func() {
 		err = acc.Close()
 		tcheck(t, err, "closing account")
+		acc.CheckClosed()
 	}()
 	defer store.Switchboard()()
 
diff --git a/webapisrv/server_test.go b/webapisrv/server_test.go
index 8e7ab36..5fb3886 100644
--- a/webapisrv/server_test.go
+++ b/webapisrv/server_test.go
@@ -76,6 +76,7 @@ func TestServer(t *testing.T) {
 	defer func() {
 		err := acc.Close()
 		log.Check(err, "closing account")
+		acc.CheckClosed()
 	}()
 
 	s := NewServer(100*1024, "/webapi/", false).(server)
diff --git a/webmail/api_test.go b/webmail/api_test.go
index 7d3f902..7919514 100644
--- a/webmail/api_test.go
+++ b/webmail/api_test.go
@@ -66,6 +66,7 @@ func TestAPI(t *testing.T) {
 	defer func() {
 		err := acc.Close()
 		pkglog.Check(err, "closing account")
+		acc.CheckClosed()
 	}()
 
 	var zerom store.Message
diff --git a/webmail/view_test.go b/webmail/view_test.go
index f8a3950..743e5d6 100644
--- a/webmail/view_test.go
+++ b/webmail/view_test.go
@@ -39,6 +39,7 @@ func TestView(t *testing.T) {
 	defer func() {
 		err := acc.Close()
 		pkglog.Check(err, "closing account")
+		acc.CheckClosed()
 	}()
 
 	api := Webmail{maxMessageSize: 1024 * 1024, cookiePath: "/"}
diff --git a/webmail/webmail_test.go b/webmail/webmail_test.go
index 90e032d..f70bce3 100644
--- a/webmail/webmail_test.go
+++ b/webmail/webmail_test.go
@@ -301,6 +301,7 @@ func TestWebmail(t *testing.T) {
 	defer func() {
 		err := acc.Close()
 		pkglog.Check(err, "closing account")
+		acc.CheckClosed()
 	}()
 
 	api := Webmail{maxMessageSize: 1024 * 1024, cookiePath: "/webmail/"}