diff --git a/store/account.go b/store/account.go
index d890733..628ce18 100644
--- a/store/account.go
+++ b/store/account.go
@@ -1081,7 +1081,9 @@ func OpenAccountDB(log mlog.Log, accountDir, accountName string) (a *Account, re
 	}
 
 	// Ensure singletons are present. Mailbox counts and total message size, Settings.
-	// Process pending MessageErase records.
+	// Process pending MessageErase records. Check that next the message ID assigned by
+	// the database does not already have a file on disk, or increase the sequence so
+	// it doesn't.
 	var mentioned bool
 	err = db.Write(context.TODO(), func(tx *bstore.Tx) error {
 		if tx.Get(&Settings{ID: 1}) == bstore.ErrAbsent {
@@ -1165,6 +1167,77 @@ func OpenAccountDB(log mlog.Log, accountDir, accountName string) (a *Account, re
 			}
 		}
 
+		// Ensure the message directories don't have a higher message ID than occurs in our
+		// database. If so, increase the next ID used for inserting a message to prevent
+		// clash during delivery.
+		last, err := bstore.QueryTx[Message](tx).SortDesc("ID").Limit(1).Get()
+		if err != nil && err != bstore.ErrAbsent {
+			return fmt.Errorf("querying last message: %v", err)
+		}
+
+		// We look in the directory where the message is stored (the id can be 0, which is fine).
+		maxDBID := last.ID
+		p := acc.MessagePath(maxDBID)
+		dir := filepath.Dir(p)
+		maxFSID := maxDBID
+		// We also try looking for the next directories that would be created for messages,
+		// until one doesn't exist anymore. We never delete these directories.
+		for {
+			np := acc.MessagePath(maxFSID + msgFilesPerDir)
+			ndir := filepath.Dir(np)
+			if _, err := os.Stat(ndir); err == nil {
+				maxFSID = (maxFSID + msgFilesPerDir) &^ (msgFilesPerDir - 1) // First ID for dir.
+				dir = ndir
+			} else if errors.Is(err, fs.ErrNotExist) {
+				break
+			} else {
+				return fmt.Errorf("stat next message directory %q: %v", ndir, err)
+			}
+		}
+		// Find highest numbered file within the directory.
+		entries, err := os.ReadDir(dir)
+		if err != nil && !errors.Is(err, fs.ErrNotExist) {
+			return fmt.Errorf("read message directory %q: %v", dir, err)
+		}
+		dirFirstID := maxFSID &^ (msgFilesPerDir - 1)
+		for _, e := range entries {
+			id, err := strconv.ParseInt(e.Name(), 10, 64)
+			if err == nil && (id < dirFirstID || id >= dirFirstID+msgFilesPerDir) {
+				err = fmt.Errorf("directory %s has message id %d outside of range [%d - %d), ignoring", dir, id, dirFirstID, dirFirstID+msgFilesPerDir)
+			}
+			if err != nil {
+				p := filepath.Join(dir, e.Name())
+				log.Errorx("unrecognized file in message directory, parsing filename as number", err, slog.String("path", p))
+				maxFSID = max(maxFSID, id)
+			}
+		}
+		// Warn if we need to increase the message ID in the database.
+		var mailboxID int64
+		if maxFSID > maxDBID {
+			log.Warn("unexpected message file with higher message id than highest id in database, moving database id sequence forward to prevent clashes during future deliveries", slog.Int64("maxdbmsgid", maxDBID), slog.Int64("maxfilemsgid", maxFSID))
+
+			mb, err := bstore.QueryTx[Mailbox](tx).Limit(1).Get()
+			if err != nil {
+				return fmt.Errorf("get a mailbox: %v", err)
+			}
+			mailboxID = mb.ID
+		}
+		for maxFSID > maxDBID {
+			// Set fields that must be non-zero.
+			m := Message{
+				UID:       ^UID(0),
+				MailboxID: mailboxID,
+			}
+			// Insert and delete to increase the sequence, silly but effective.
+			if err := tx.Insert(&m); err != nil {
+				return fmt.Errorf("inserting message to increase id: %v", err)
+			}
+			if err := tx.Delete(&m); err != nil {
+				return fmt.Errorf("deleting message after increasing id: %v", err)
+			}
+			maxDBID = m.ID
+		}
+
 		return nil
 	})
 	if err != nil {
@@ -3053,6 +3126,7 @@ func OpenEmail(log mlog.Log, email string, checkLoginDisabled bool) (*Account, s
 
 // 64 characters, must be power of 2 for MessagePath
 const msgDirChars = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ-_"
+const msgFilesPerDir = 8 * 1024
 
 // MessagePath returns the filename of the on-disk filename, relative to the
 // containing directory such as <account>/msg or queue.
diff --git a/store/account_test.go b/store/account_test.go
index c8f3713..fbf480d 100644
--- a/store/account_test.go
+++ b/store/account_test.go
@@ -312,3 +312,90 @@ test
 
 	// todo: test the SMTPMailFrom and VerifiedDomains rule.
 }
+
+// Check that opening an account forwards the Message.ID used for new additions if
+// message files already exist in the file system.
+func TestNextMessageID(t *testing.T) {
+	log := mlog.New("store", nil)
+	os.RemoveAll("../testdata/store/data")
+	mox.ConfigStaticPath = filepath.FromSlash("../testdata/store/mox.conf")
+	mox.MustLoadConfig(true, false)
+
+	// Ensure account exists.
+	acc, err := OpenAccount(log, "mjl", false)
+	tcheck(t, err, "open account")
+	err = acc.Close()
+	tcheck(t, err, "closing account")
+	acc = nil
+
+	// Create file on disk to occupy the first Message.ID that would otherwise be used for deliveries..
+	msgData := []byte("a: b\r\n\r\ntest\r\n")
+	msgDir := filepath.FromSlash("../testdata/store/data/accounts/mjl/msg")
+	os.MkdirAll(filepath.Join(msgDir, "a"), 0700)
+	msgPath := filepath.Join(msgDir, "a", "1")
+	err = os.WriteFile(msgPath, msgData, 0700)
+	tcheck(t, err, "write message file")
+
+	msgPathBogus := filepath.Join(msgDir, "a", "bogus")
+	err = os.WriteFile(msgPathBogus, []byte("test"), 0700)
+	msgPathBadID := filepath.Join(msgDir, "a", "10000") // Out of range.
+	err = os.WriteFile(msgPathBadID, []byte("test"), 0700)
+
+	// Open account. This should increase the next message ID.
+	acc, err = OpenAccount(log, "mjl", false)
+	tcheck(t, err, "open account")
+	defer Switchboard()()
+
+	// Deliver a message. It should get ID 2.
+	mf, err := CreateMessageTemp(log, "account-test")
+	tcheck(t, err, "creating temp message file")
+	_, err = mf.Write(msgData)
+	tcheck(t, err, "write file")
+	defer CloseRemoveTempFile(log, mf, "temp message file")
+	m := Message{
+		Size: int64(len(msgData)),
+	}
+	err = acc.DeliverMailbox(log, "Inbox", &m, mf)
+	tcheck(t, err, "deliver mailbox")
+	if m.ID != 2 {
+		t.Fatalf("got message id %d, expected 2", m.ID)
+	}
+
+	// Ensure account consistency check won't complain.
+	err = os.Remove(msgPath)
+	tcheck(t, err, "removing message path")
+	err = os.Remove(msgPathBogus)
+	tcheck(t, err, "removing message path")
+	err = os.Remove(msgPathBadID)
+	tcheck(t, err, "removing message path")
+
+	err = acc.Close()
+	tcheck(t, err, "closing account")
+
+	// Try again, but also create next message directory, but no file.
+	os.MkdirAll(filepath.Join(msgDir, "b"), 0700)
+	os.MkdirAll(filepath.Join(msgDir, "d"), 0700) // Not used.
+
+	// Open account again, increasing next message ID.
+	acc, err = OpenAccount(log, "mjl", false)
+	tcheck(t, err, "open account")
+
+	// Deliver a message. It should get ID 8*1024+1.
+	mf, err = CreateMessageTemp(log, "account-test")
+	tcheck(t, err, "creating temp message file")
+	_, err = mf.Write(msgData)
+	tcheck(t, err, "write file")
+	defer CloseRemoveTempFile(log, mf, "temp message file")
+	m = Message{
+		Size: int64(len(msgData)),
+	}
+	err = acc.DeliverMailbox(log, "Inbox", &m, mf)
+	tcheck(t, err, "deliver mailbox")
+	if m.ID != 8*1024+1 {
+		t.Fatalf("got message id %d, expected 8*1024+1", m.ID)
+	}
+
+	err = acc.Close()
+	tcheck(t, err, "closing account")
+	acc.WaitClosed()
+}