What are you tring to do? you can pop emails with Outlook directly from your sendmail server (no need for exchange), but if for some reason you need the exchange server, then consider this:
Exchange has a setting which allows all outgoing mail to be forwarded to your sendmail gateway.
Now for your example.com domain, you should make a mailertable entry (/etc/mail/mailertable) for like this:
This way all inbound mail would be forwarded to your exchange server.
If you want only certain users to use the exchange server, then use /etc/mail/virtusertable:
And make sure their mailboxes now reside on exchange server.