Linux machines come with POP3/SMTP abilities. The lowest cost Microsoft product to just provide SMTP service is Windows-2000, if IIS-5 is installed.

If POP and SMTP service is all you need, then Microsoft Exchange is a waste of money. If Exchange seems needed, consider also Unix-based alternatives such as HP Open Mail. The big problems with Microsoft Exchange are the version upgrades they keep forcing on you and the requirement (because Microsoft designed it that way) of being in a domain controlled by an NT (or 2000) server, forcing up your expenses beyond what is reasonable for the job.