Hahah, omniwall sucks, so does any other type of gateway for exchange.
It's easy to setup your POP3 exchange to avoid relay, here are the steps :
Start exchange administrator,
Under your domain name, click "connections"
you will see on your right "Internet Mail service(servername)" - double click that
Click on Routing : make sure you have
1. Reroute incoming SMTP mail (required for POP3/IMAP4 support)
2. Add a inbound route that is your server.domain.com (same one as the mail server)
3. Click on "routing restrictions"
HERE IS THE stupid part..
You will see two top checkboxes. Click both of those, leave the rest, hit OK, and close out and reboot.
It's retarded, but it keeps from your server being an open relay.