This service has the email routed to their mail servers and is filtered for spam then it is sent to your mail server which can save you bandwidth cost. It is easy to implement and allows spam filtering rules to be updated easily.

A challenge/response spam filtering service that works with your current email address. Allows pre-configuration of approved senders and gives you access to unapproved email.

Their site states "The Demon spam-filtering service is a free-to-use, opt-in/opt-out service that applies sophisticated filtering technology on our mail servers to attempt to identify spam e-mails. Once the service is activated, e-mails classified as spam are diverted to separate mail servers, where they can still be accessed by you for seven days, after which, they will be automatically deleted."

Filters spam with challenge/response filtering. It allows you to keep your same email address and has several available plans.