[Mimedefang] Blocking e-mail where the To does not match the envelope To

[Jon Kenoyer]

Hi we recently got a pretty nasty pornographic spam.  Below is the
header information :
for <otsiweb at otsiweb.com>; Thu, 29 Aug 2002 07:39:01 -0700 (PDT)
	(envelope-from dbandeb at earthlink.com)
To: dbandeb at earthlink.com

As you can see the To does not match the actual for indicated in the
envelope.
Could I add a rule to discard e-mails where the To does not match the
actual recipients.  If so could someone post a quick sample?
Are there cases where this could block legitimate e-mails?