[enhancement]: Garbage collection

[nomadturk]

Which feature or improvement would you like to request?

After a domain is removed, settings and DKIM keys pointing to that old domain are still kept in the database.
These should be removed, either periodically or right away.

On busy servers, after a while, these will start causing performance issues due to needless lookups as well.

Also, currently there are no checks for existing email addresses while removing a domain.
Either we shouldn't be able to remove the domain OR must delete the mailboxes along with the domain.

Is your feature request related to a problem?

I'm having a problem with...

Code of Conduct

• I agree to follow this project's Code of Conduct