VMServer and email

Ok, in looking over some spare equipment and hardware at work, I have put together a pretty good server box that I am have put VMServer on and now want to look at moving part of our email over to. Most of the people who get email only get a basic email account while a select few get an Exchange account. For those who get basic email, we are using iMail but we may consider moving them over to something else.

Anybody have any ideas on a good open source email package that runs good on vmserver? The server has a Pent. 4 3ghz processer with a 1.25 GB of memory which we wil upgrade later. It has a 300GB SATA drive pluse we can add more later so we are in pretty good shape hardware wise. Now we just need to look for a good email package.

dj