[Exim] Antivirus for Exim/SMTP?

Hi,

you may have already had this question, but basically I'd like to know if
there is any antivirus that would exist for Linux and that would work on the
email server straight.

For instance I received once a message back from a server cos' the message I
sent had a virus (the guy I sent this message to tehrefore didn't get it).

Would this 'antivirus' be expensive or free??

cheers
Guillaume