At the risk of not supporting/recomending a Microsoft Product for the Microsoft Platform, why not use a Shareware/Freeware product. http://www.zdnet.com/downloads search found 52 POP3 servers 85% of which work with NT. the iMAIL Server for NT works well with IIS.
I know this is not exactly what you asked for, but should give you a numer pf products to try which cost less than Exchange.
Note if you are supporting 50 or more users, bite the bullet and get MS Exchange, or learn Linux and use Sendmail.