how to kill base64 emails?

[Jeff Earickson]

Gang,
   I've noticed lately that a lot of spam is base64 encoded,
and I've also noticed:

X-MIME-Autoconverted: from base64 to 8bit...

in my mail headers lately (4.28 era).  What is this about?
This may relates to my howl about an uptick in spam with 4.28
that I had this morning.  Maybe due to recent changes in
the MIME code?

Is there a quick way in MailScanner to mark base64 encoded
messages as spam?  Is this a bright idea to do, if I can?

Jeff Earickson
Colby College