I'll second the recommendation for djb software. http://perl.plover.com/yak/qmail/ http://perl.plover.com/yak/qmail/ has slides from a presentation about qmail

I'll second the recommendation for djb software. http://perl.plover.com/yak/qmail/ http://perl.plover.com/yak/qmail/ has slides from a presentation about qmail internals.