quoted-printable, 7bit, 8bit Content-Transfer-Encoding issues
Postfix Users - On DKIM and Content-Transfer-Encoding. Setup a dummy smtpd server that will force conversion to quoted-printable (universally supported?)
Things to try to reduce spam
Comment on LWN mentions on requiring TLS. Looked at some spam I received, and it was delivered via TLS -- not sure how effective this would be.
Mail Avenger can reject e-mail before it's received, and can do things like checking the OS of the sender (i.e. to reject anything Windows).