CTS provides pre-delivery scanning of all incoming e-mail for spam: unsolicited bulk e-mail, junk advertising, e-mail scams, etc.
All mail is put through a series of tests on the mail server to determine what level of spam the message will score. More weight is given to 'delivery' information that accompanies each message (you don't normally see this when you read your mail). The bulk mailing tools used by spammers tend to leave their signatures here, and the filters evaluate whether the information is consistent, realistic, or has known bulk-mailer characteristics. The tally of all positive and negative scores is the spam score of the message.
For more information about the spam tests listed in your email headers, and what they mean, please see http://spamassassin.apache.org/tests.html
Users are automatically enrolled in our Mailguard service, which by default, quarantines all e-mail beyond a certain spam score threshold (setting: high). Users can see the quarantined mail by logging into the web interface and reviewing the questionable items. They can also release any mis-identified items for re-delivery. See the 'Mailguard' section below for more details.
Users may choose to instead filter all mail based on our previous method of spam-tagged subjects. They will need to login to Mailguard to effect this change (setting: medium). See the 'Client-side Spam Filtering' section below for more details.