Given an email like firstname.lastname@example.org .
It is necessary to define the incoming mail server (address and port) for reading the letter. It is desirable to specify a username.
email@example.com should issue
mail.ekotek.info:110 username name+ekotek.info
For the latter, you can not check. Brought only to show that the names for postal addresses are taken not before the @ symbol. And I doubt that this is even possible for the latter (name definition).
BUT: The Gmail service defines the domain perfectly.
I would venture to suggest that there is an enumeration of common names (pop, pop3, mail, relay, etc.) with a check on port 110.