Email sent between mail servers is always sent on port 25. This is not configurable and is the same for all mail servers. If you change the SMTP port to anything other than 25, other servers will not be able to send you email. Unless, you have set up some kind of portforwarding.
If your ISP has blocked port 25 for incoming traffic in their firewall, or you can't use port 25 for some reason, there are only two things you can do: