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.