IMP 3 and Multiple domains

[imp@redstarhosting.com]

I have IMP 3 working and it is fast and feature rich.  Hats off to the 
developers.  Great example of team work and talent.

A question.  I have a primary mail server that hosts multiple domains.  IMP is 
on its own server but needs to be able to understand that multiple domains 
exist.  eg. When john logs in, he is with domain1.com but when mark logs in, he 
is associated with domain2.com.  This is so the from address and such is 
correct.

How can this be done?  I want to make it as easy as possible for the users.

Thanks

Mark