The local part is everything before.
Some domains have owners.
It is up to the owner of heaven.af.mil to say how mail messages will be delivered to addresses with domain part heaven.af.mil . AF.Mil all refer to the same domain. The empty string cannot be used as a recipient address. It can be used as a sender address so that the real sender doesn't receive bounces. The qmail system allows several further types of addresses in mail envelopes.