It is no good idea to allow direct access to Exchange servers from the Internet without additional protection. Using username/password to access mailboxes is certainly not enough! In the past we have seen many attacks even using zero day exploits….
You have hosted a website using ApplicGate. This web site has a form that posts data. Now this data should be sent via email to somebody.
If you would like to secure UDP traffic via unsecure networks or network segments you can add authentication and encryption easily.