This reverts merge request !1812

As the regex condition changed, the error message for invalid did too.
Lanuage files for en/de/es are now state the new rules for a valid selector.

Changed the regex pattern for dkim_selectors again. We now follow
the DKIMCore technical specification as stated on the website:

A selector is a string of no more than 63 lower-case alphanumeric characters (a-z or 0-9) followed by a period “.”, followed by another string of no more than 63 lower-case alphanumeric characters.

The dkim_select can start with a number, but has to contain atleast
1 letter and has to be between 1 and 63 characters long.

before the regex matched empty entries and entries starting with a number.

add documentation and example for mail_relay_domain_delete function #6399

This could be refactorred into a function ...

Fix issue 6362