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'  ...