Spam Checker utilizes SpamAssassin in order to check what tests are performed when spam filtering based on content:
http://en.wikipedia.org/wiki/Spamassassin
We
rely on SpamAssassin to perform tests because the criteria that is set
for whether a mailing is marked as spam the best resource to find out
what a particular score means would be to go to the source of the tests
that were performed.
You can view tests performed here:
http://spamassassin.apache.org/tests.html
Any questions or confusion about what is used should be directed to this site.
