I have a Postfix server filtering emails for an Exchange 2003 server. From the firewall, all incoming emails are routed through Postfix, gets filtered before it is delivered to the Exchange 2003 server. All outbound emails are relayed to the Exchange server first and then it gets relayed to the Postfix server then out to the internet.
Inside the network, I have Outlook clients connecting to the Exchange server via MAPI.
However, I have some users who are outside the network and are using the Exchange server to send out emails via port 25 (NAT redirection). These users use Outlook Express, Thunderbird, and various mobile devices. I want to change this to a non-standard port (e.g. 465) and use TLS/SSL to secure the communication between end-client and the server. How do I do this without changing any configuration for the users inside the network?