Why won't SpamAssassin tag a message subject?
Symptom: I configured SpamAssassin to add the spam score to the subject, but it won’t do it.
Cause: When you run SpamAssassin under MIMEDefang, SpamAssassin cannot change the message in any way.
If you want to add any spam-related headers, you will have to do this in your rules file (mimedefang-filter).
Most people seem to do this in filter_end (after all parts have been checked for viruses).
The sample filter has code to call SA and add an X-Spam-Score: header to messages SA tags as spam.
If you want to change the subject, use something like this in filter_end:
action_change_header('Subject', "***SPAM*** $Subject");