Virus filter on mail server?

I want to run a mail server on a windows based machine for a medium to large size number of clients. I also want to be able to filter the emails against viruses for added safety. Anybody know of any good mail servers that either have virus filtering as a feature or as an add-on? free is preferrable, but commercial is also an option.

I quite like gfi mailsecurity. You can download a 30 day trial, but the one virus filter remains even after 30 days.

I would use either
1. mcaffee webshield appliance this is nice as it will scan e-mail, ftp, http, pop3, and smtp you place it right after your firewall in bridge mode it also does SPAM blacklists and whitelists it operating system is redhat but all configuration is done through a web browser very nice and easy to use not suseptible to all throughs microsoft viruses. ( hard to explain that the virus protection got infected by a new virus hehe)

2. CA makes a descent product called secure content manager which you would load on a seperate box then the mail server and have it scan then push to the mail server or out to the internet ( need to change your NAT for smtp to this box ) or load on the mail server and change the port exchange listens on. also does content management and blacklists for spam.

these are like corporate software packages I recommend these because to me a medium size e-mail system is 250 users +

I use PANDA on our school systems. Easy to administer and configure and the filter setup (SPAM/PROFANITY etc) is straightforward too. Of course its a combination AV and filter so you get both. I found it to be much cheaper than competitor products (CA, Trend, McAffee and Norton). Oh yes, it will background scan server mailboxes too so it will find infected files that are already in users mail folders (it found some on our exchange box that Trend and CA missed!).