Given an emailof the form [email protected].
You need to define an incoming mail server (address and port) to read the email. It is desirable to specify a username.
You don’t have to check for the latter. 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 nicely.
Answer 1, authority 100%
I would venture to suggest that there is a search of common names (pop, pop3, mail, relay, etc.) with a check on port 110.