Aliases and users with the same address are not checked for

[ddavness]

Right now you can create an user, let's say [email protected] and also create an alias [email protected] (and vice-versa).

Reproduction:

• Create an user via the Users control panel. Let's call it [email protected].
• Create an alias [email protected]. It shares the same address as the user.
• • Expected behavior: The alias is not created (and the endpoint returns an error), because it's address already exists as an user.
• • Actual behavior: The alias is created like nothing happened.

Proposed Resolution: Check in add_mail_user() and add_mail_alias() whether the target address exists on the aliases and users table, respectively.

[A3K]

This is desirable and working behaviour IMO allows the person in charge of the address to receive and manage emails while also forwarding to anyone else who needs a copy.