Virus scanner for e-mail gateway at ISP

We are providing POP3 e-mail service and face the frequent question from customers about a possibility to scan incoming and outgoing e-mails, including attachments (.txt, .doc, .ppt, etc.), at our mail-server gateway. It would be an excellent way tokeep customers from deserting to cable providers.
Our question:
Does such a SW product exist? If so, for which SW Platform and at which price?