Email Login w/Domain Alias

One of the client accounts I’m about to migrate has about 40 mailbox users spread around the country all using the user@domain.tld style username. This by itself is no big deal, VM handles this well.

The challenge in this case is that the account has a couple domain aliases and some mailbox users login using user@domain1.tld an others use user@domain2.tld.

My experiments so far make me think that only the main domain name can be used in the login, as I can’t seem to login using the alias user@domain2.tld. Am I wrong about this or do I need to get all users to login using the main domain name?