[Exim] Help with aliasiing

Can someone pls help with an aliases question.
I have eg two domains domaina.com and domainb.com.
There are two users on the system user1 and user2.
How can I set up aliasing so that user1@??? will receive remote mail
while user1@??? will not work.
I have checked and am using the include_domain function under the alias
sectin in the config file but no joy. User1 can receive mail on any of the
domains.

Pls help.

Regards
Leon Botes