I wrote earlier in the year of my migration from Sendmail to QMail – and have never looked back. I am happily blocking a multitude of IP addresses and blocks, and in conjunction with SpamAssassin have vastly reduced the amount of spam I and my clients have to wrestle with.
My situtation is slightly different from that of web host, as outside of my research work I remotely manage web and other servers for clients (i.e. numerous servers for unrelated companies generally on their own premises or data centers). Outside of a handful among them running Lotus Domino or Microsoft Exchange servers using native SMTP, the majority have migrated to QMail as their MTA.
I particularly like the ease of blocking IPs in Qmail, however, I wanted to end the year with some links to well written how-to’s for the most popular MTAs.
Postfix – http://www.postfix.org/spam.html
Sendmail – http://www.sendmail.org/m4/anti_spam.html