pop3 and smtp accounts on exchange2000

Hi all I have a problem. I have a small buisness server 2000 running exchange 2000. I have two seperate email domains parag.com and paraa.com. one has an mx record pointing to my exchange server the other is a pop3 account. My users use mail from both accounts. I need to have the users send from domain g and from domain a. we use outlook 2000 any thoughts would be appreciated