Because of (Belgian) providers limiting SMTP (port 25) traffic to their own servers, makes it necessary for us (as an 3rd party ISP) to allow our customers to use our own mailserver for all outgoing mail. To circumvent this limitation, we've configured the mailserver to listen on port 26 as well.

For our customers with only a few mail clients (so no local mailserver), this is usually not a problem. A quick change of the SMTP port, solves their issues. For people that have a catch-all mailbox (and a local mailserver), the mailserver itself usually allows the change.

Of course, some of our clients have Exchange servers, and there, the change was not that obvious.
After a bit of a search, we've found this article: http://support.microsoft.com/kb/274842/.

Changing outgoing SMTP port (for smarthosts)

The brief solution to our problem (changing the outgoing SMTP port to 26). Screenshots are from a Exchange 2003, though they should be similar in Exchange 2000 (don't know about Exchange 2007).

First start up the Exchange System Manager, then navigate to Servers - your server name - Protocols - SMTP and right-click the Default SMTP virtual server, then choose Properties.

Switch to the Delivery tab, and hit the Outbound connections button.

A new dialog pops up, where you can change the TCP Port to something else (in our case, it would be 26):

There we went. The changes were effective immediately, and nicely circumvented the limitations posed by the customer's ISP.