Suggest you use server-side rather than client-side filtering.
There are many external services available to assist the server-side to rid itself of passing on garbage mail to the end-user. Clam AV, Amavis, SpamAssassin, Postgrey are all free and do a brilliant job. Even the MTA Postfix can check against valid sender DNS names before delivering your mail.
There is no excuse. Just proves any dope can set up a mail server inside an ISP and pass the buck onto the end-user.