You want to run a MailScanner using sendmail and you only want it to relay to an other server (Exchange, Domino, Groupwise...). Here is the best way to do it:
(The brackets tell sendmail to bypass DNS resolution.)
This tells sendmail to push the mail off to the Exchange server, but crucially, it does this AFTER MailScanner has done its work.
The user then picks up their now cleaned mail from Exchange. You should also configure your exchange server so that it uses your MailScanner server for Smart Host (don’t forget to create rulesets accordingly.