First of all, here is something that niggles me. Whenever someone opens thier eyes to an OS beyond Windows, the first one they hit is always Linux. Heres an idea, forget Linux. Look into FreeBSD. I won't bother listing advantages and disadvantages of FreeBSD over other operating systems, but I will say that there are many.
The email server you use depends on the number of users you will be serving. If it is only a handful I would use qmail. However, if you are talking about thousands of users, perhaps Courier would be more suitable. Both are available for FreeBSD and Linux.
Please, don't disregard FreeBSD just because you may not have heard of it before. The only reason it is less popular than Linux is because people either don't know about it, or they refuse to try it (or they refuse to admit it is better than Linux).
Not intending to start a flame war here, but I thought it wouldn't be a bad idea to pimp FreeBSD before everyone starts posting Linux-oriented comments.