handle case cannot parse mailbox_id

email_handler.py

@@ -874,8 +874,13 @@ def handle_bounce(contact: Contact, alias: Alias, msg: Message, user: User):
             file_path, BytesIO(orig_msg.as_bytes()), random_name
         )
         # <<< END Store the bounced email >>>
-
+        try:
             mailbox_id = int(orig_msg[_MAILBOX_ID_HEADER])
+        except TypeError:
+            LOG.warning("cannot parse mailbox from %s", orig_msg[_MAILBOX_ID_HEADER])
+            # use the alias default mailbox
+            mailbox = alias.mailbox
+        else:
             mailbox = Mailbox.get(mailbox_id)
             if not mailbox or mailbox.user_id != user.id:
                 LOG.exception(