Missing validation for mail_domain and mail_transport

There is currently no validator function to verify whether an Email Domain or Email Routing entry with the same domain name already exists when creating a new entry.

We should implement a check to detect such cases and display an appropriate error message to warn the user, as this could potentially disrupt the mail functionality for the domain.

More info in the discussion thread in the forums: https://forum.howtoforge.com/threads/check-for-mail-routing-mail-domain.93330