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I still make use of Windows 2000 for a mail server.
I use hMailServer along with SQL Server 2005 on a Windows 2000 box. Why Win2K? Well, for no other reason than I could.
The box itself is pretty low end (2GB RAM max, 2.2Ghz Athlon XP processor, consumer-grade parts, etc.) but it manages to handle 20,000 - 60,000 pieces of spam per day, reporting about 99.98% of spam to SpamCop automagically. IMHO, not too shabby for a machine that has been chugging along for six years now with all of about twenty minutes of downtime (when I had to swap out a BFG nVidia 5500OC video card whose fan practically melted off of it… suffice it to say, the backup was an identical sister card with the stock heatsink replaced with a massive passive heatsink).
Of course, not only is the box behind a very strongly configured router/firewall, but it also has its TCP/IP and UDP ports manually locked down so only two ports - 25 & 110 - are actually open. The rest refuse all communication.