1. A reverse lookup is carried out on the sender to establish if the sender has an authentic email address and it is present on the sending SMTP mail server. If this does not exist it is returned to the sender as spam.
2. Received messages are classified on a points system using Spam Assassin. They are given a messages score, if it exceeds a predetermined very high are considered high spam and value the message will be returned to the sender with a message stating that it has failed and a number to put in the header if it is a genuine email.
3. Any messages that scores between 5 points and 15 are considered low spam. These messages are flagged with special message headers indicating that they are "probably spam" and are passed on to your server. You can create filter rules to see these messages and route them to a spam folder. The reason we do it this way is because no spam filter is perfect. But we deal with the fact that messages are missed or wrongly identified as spam in a way that helps to ensure that you don't lose any email.