mail domain alias allowed when shouldn't be
When adding a mail domain alias there should be a check for existing addresses (mailboxes, forwards, aliases and catchall) in that domain and not allow creating the domain alias if found.
Checks in the other direction exist for mailboxes/forwards/alaises, but also missing for catchall. (Ie. you cannot create a mailbox for a domain which is setup as an alias, but you can create a catchall.)