I'm using fast hosts that comes with some custom pop/smtp setup called matrix. It has a fairly simple web interface to setup accounts and mail forwarding and aliases but has NO webmail, imap, spam filtering, virus checking etc..
I'm in two minds as to whether I replace this setup with something better like mailenable which is free and I believe includes spam filtering or installing some kind of smtp spam proxy.
I've seen the likes of spam assassin but really I want an affordable commercial solution that is unlimited in terms of domains and users. I don't feel confident playing around with things like spam assassin.
Additionally virus protection would be a bonus but not a requirement as I can manage this fairly effectively by blocking certain attachments and on the client. SPAM is a real issue with 1000's of them!