hi,
the best mail server is postfix mta from
www.postfix.org
there are a plenty of howtos with respect this mta and very easily configurable also as it is easily human readable. One can easily interface this with antivirus software from
www.openantivirus.org or from f-prot which gives single home alone version free and various anti spam tools like amavis, spamassassin, etc
try this this realy works I have currently running around 8,000+ mail baoxes with this setup
regards
shann