Some relevant pointers about Mail servers and Mail filtering
MTA - Mail Servers
milter API
milter.org -
An interactive catalog of sendmail mail filters
Virus filtering
Spam filtering
Greylisting - The Next Step in the Spam Control War - Evan Harris web site about Greylisting, and his implementation (relaydelay). This is the reference site about greylisting
SURBL - Spam
URI Blocklist
Books
Bruno Rasle and Frédéric Aoun -
Halte au Spam - a very good french book about spam.
Alternative libmilter - This is a modified libmilter using a pool of workers instead of one thread per SMTP connection. This allows to drastically reduce the number of threads in the filter : interesting if your server handles lots of simultaneous connections. This implementation of libmilter was integrated on sendmail 8.14.0 and received an
Innovative Open Source Contribution award from sendmail.
Vigilog - a set of perl scripts to process Cisco Routers log files and detect scans.
j-sophosd a daemon virus scanner using Sophos savi library (not included) to use it with ze-filter release 1.7 and newers. This is a prototype.