Change 'incoming' smtp port

I have an OSX server running on an old mini (snow leopard server, anything newer does not run on this mini). It does not do much, just VPS and mail relay. Since I am on a lot of different locations I use my own server as my outgoing mail server so I don't need a long list of ISP's smtp servers on my laptops etc. The server is set to relay the mail through my ISP.
All is well, except when I am on a location where port 25 is blocked. Then of course I cannot reach my own server and I will still have to use the local ISP's smtp.
I'm guessing all I have to do to change this is use a different port. I know where to set this on my laptops and phones, but of course I will have to tell the server to listen on another port as well. But where? And what port-number would you suggest?

Alrescha said:

Before you change port 25, you might consider adding port 587 (email submission) to your forwarding list.

A.

Click to expand...

Not really sure what that means... What forwarding list and where do I find it?

Ah, all right, that forwarding list...
Ports 587 and 465 are already routed. That does not help. Apparently either my outgoing mail only uses 25 (mail.app set to default where all three ports are listed) or the server is only listening on that port.
Thanks for the tip though!

A