I've been using a Pentium III 550MHz machine as my mail server for a long time. I installed FreeBSD 4.4R on it when I got it, and other than upgrading the system to Ultra160 disks in 2003 or so, and the OS often, it has been my faithful mail server for about a decade. It is now running the latest FreeBSD 7.x-stable.
% kenv | grep reldate
smbios.bios.reldate="01/15/99"
It has only been in the past 6 months that I've thought about replacing it. It just can't handle the load of spam I get, plus spamassassin and a couple other anti-spam counter-measures. Each message takes about 5 seconds to process on this old iron, and once I went north of 10k messages per day, I noticed the queues starting to grow... While I've also put in place some cheaper rejection rules that have helped bring the queue lengths down. It is slated to be replaced by a harperton 3.4GHz box as soon as I can find the cycles to migrate everything over to a jail on it.
Prior to using this machine, my mail server was a 486DX2/66 with 32M of RAM that I used from about 1995-2002....
"If it ain't broke, don't fix it."
Warner