[Exim] user addresses and domain name additions

I have user1 with an address is user1@??? as well as other local
users that "belong" to domain1.com

user2 has an address of user2@???, as well as other local users tat
belong to domain2.com, etc etc.

I am using lsearch files to list all users in my virtual domains. It is
working flawlessly. However, is there someway I can specify that user1
ALWAYS has an address of user1@???, no matter what they have
configured in their MUA? I am also using webmail and I don't want to
manually edit the preferences for each user as they are added, I would like
to include them in a file and always have their correct domain added to
their username.

Man that sounds like gibberish, but I can't think of a better way to
explain it.