[Exim] more domains

Hi,

I'd like to make a server to be mx for three domains.
That wouldn't be a problem, but I want to be able to determine which user
can use which domain.

For example if I have

aaaa.com
bbbb.com
cccc.com

And my example users are Mary and John.
I want to allow Mary to receive mail to mary@??? but I dont want to
allow her to receive mail to the other two.
I want to allow John to receive mail to john@??? and john@??? but
I dont want to allow him to receive mail to john@???.

Could you help?

Thanks
Mike Litzler