The necessary evolution of mail servers
In a comment on my Legend of Debian post, Chris Wage wrote in part:
Most of the servers I run are: webservers, mailservers, CVS servers, etc. These are things for which well-established stable software has existed for years. I don't need bleeding-edge software to do them. I need stable representatives of that software that are supported by security updates but don't otherwise change.
I have to disagree with this in the case of mail servers.
Unless you actively enjoy getting spammed your mail server software needs to be upgraded on a regular basis, because spammers evolve their techniques all of the time. One of my major issues with Debian Woody is that by the end of its life, its default mailserver (Exim 3) is clearly not adequate to the job of stopping spam.
This means that if you do spam filtering at all, you're going to need regular mail server software upgrades in some form. (This also means that you're going to need to evaluate upgrades if your operating system vendor doesn't deliver regular ones.)
Virus authors also evolve their tricks all the time, so people doing virus filtering need to think about this issue too.