I have become a real fan of Exchange 2007 - which may surprise some readers as I've previously been a non-fan. But with Exchange 2007, I'm finding that Microsoft has done something really pretty cool. Exchange 2007 is not a little mail system - it is an enterprise messaging solution (apologies for driving off into marketing speak, but the distinction is, IMHO, important.
At home, I've been running MailTraq, a nice SME mail server. It does mail as well as NNTP and also provides both mailing lists and web mail. Well I used to run Mailtraq - I now am running Exchange 2007 Beta 2 and it rocks.
One of the interesting features of Exchange 2007 is that, in production, it's 64-bit only. There are to be no 32-bit versions (for production), although for testing and training, Microsoft will provide a 32-bit version. This requirement is being made for a combination of performance and security. But in making the move, there are some backwards compatibility issues - not least of which is that Exchange Server 2003 will NOT run on 64-bit. Microsoft has issued an KB article, Why Exchange Server 2003 will not run on the x64-based versions of Windows Server 2003 to explain why. The answer, simply, is that Exchange 2007 installs a 32-bit IFS driver which for reasons not explained has not be recompiled into 64-bit (and is not signed). Thus it won't run on 64-bit Windows Server systems that require all drivers to be both native 64-bit and signed.