If handling email is part of your life (or is likely to be in the future), you have probably enjoyed the descriptions of spamd, tarpitting, and greylisting in this book. If you want a little more background information than what you find in the relevant RFCs, the following documents and web resources provide it.
Greylisting.org (http://www.greylisting.org/) has a useful collection of greylisting-related articles and other information about greylisting and SMTP in general.
Evan Harris’s “The Next Step in the Spam Control War: Greylisting” (http://greylisting.org/articles/whitepaper.shtml) is the original greylisting paper.
Bob Beck’s “OpenBSD spamd—greylisting and beyond” (http://www.ualberta.ca/~beck/nycbug06/spamd/) is an NYCBUG presentation that explains how
spamdworks, leading up to a description ofspamd’s role in University of Alberta’s infrastructure. (Note that much of the “future work” mentioned in the presentation has already been implemented.)“Effective spam and malware countermeasures,” http://home.nuug.no/~peter/foss-aalborg2008/effective-countermeasures.pdf, originally my BSDCan 2007 paper with some updates, includes a best practice description of how to use greylisting,
spamd, and various other free tools and OpenBSD to successfully fight spam and malware in your network.