A friend of mine using (I believe)
qmail has it configured to reject, as SPAM, any incoming Email with a Date: header that is too far from real time. I can't figure out how to do this with sendmail. How do you do this?
Consider integrating something like MailScanner which can utilize SpamAssassin, ClamAV, and other external tools for detecting spam, viruses, and other nasties.
Also, you probably only want to use a date too far in the future or past as part of a spam score rather than rejecting it outright. You risk catching legitimate senders with misconfigured systems or mail clients in this way.