We are running Exchange 2003 and would like to enforce secure access by POP clients from the outside. For POP, it was simple, just select require SSL/TLS under Access Control and Require Secure Channel under Secure communication.
If we do the same for the SMTP server, including turning off anonymous access, all incoming emails are disabled. How do you force/require email clients to authenticate and use TLS, but still allow other SMTP servers on the internet to connect and deliver incoming mail normally (i.e. no authentication or SSL)?