SpamAssassin Starting to Let Noticeably More Spam Through

I have been using SpamAssassin for a few years. Until recently it caught almost all spam. Starting within the last few weeks, more spam is leaking through.

The good news is that of what leaks through, Outlook 2003 spam filters seem to recognise it and moves the mail into my "junk" folder in Outlook. As the "junk" folder exists as a file on the mail server I am running SpamAssassin's Bayesian learning program, spam-learn; however, the onslaught continues.

The spammers are clearly getting more cleaver in how they consruct their mail to slip through the nets.