I am not a programmer by any means but I know through perl regular expressions one can write such a rule, but so far I am not able to write such a rule.
Lots of rule-writing help available on the SA users list: <http://wiki.apache.org/spamassassin/MailingLists> You could also look at the wiki pages about rules: <http://wiki.apache.org/spamassassin/CategoryRules>