MailScanner and Exchange 5.5

[Steve Roy-Wojciechowski]

I am setting up a  MailScanner system that will sit infront of an
Exchange 5.5 server.  I had hoped to use milter-ahead but exchange 5.5
blindly accepts mail for the domain without first checking the user.
I was wanting incoming mail to the mailscanner  machine to be checked
by exchange and dropped at the incoming point if the user/mailbox
doesn't exist. I am using sendmail on the Linux/MS machine with mail
being forwarded via a mailertable rule.  Is there another way of
accomplishing this with either sendmail or mailscanner or even on
exchange?
There are approx 100 email addresses.  My client will be upgrading to
exchange 2003 sometime, but not in the near future.
Thanks

Steve