How will you deal with the problem, that one user can flood your
SMTP Server with tousends of emails within 10-20 minutes?
Virus filtering
Rate limit (+ script to auto terminate user) and smtp auth on outbounds
Separate inbound and outbound smtp relay. Don't let your inbound MX
relay for your dialup pool (some trojans take the rDNS name / hostname
of the infected box and do nslookup -q=mx domainname)
Ask AOL for an scomp@aol.net feed - a lot of these trojan spams seem to
target AOL users.