The majority of spam is sent by compromised zombies. Few (if any) of those rogue programs implement the full SMTP command set. More commands == larger code == easier detection. Because of this, the milter-greylist was written. What it does is, for every incoming message, an initial "temporary" error will be returned. Full blown MTA's handle this error invisibly as part of normal operations. It won't stop all spam, but it'll probably clean up most of your incoming nastiness.