That's a big question.  Read through the product descriptions for the big answer.

In summary, though: we set your MX record so that all of your email is routed to one of the MXGuardian servers.  There it is processed through many layers of sophisticated detection to identify and classify each message with a spam score.  As a result, each message is rejected, dropped, tagged as suspect or processed without modification.  The successful messages are then subjected to two or more virus filters where they are cleaned (when possible) or rendered harmless and tagged before finally being forwarded to your mail server.