Are you trying to send or receive on an alternative port? In either case, how are other mailservers going to know what port to use?
Have you asked your ISP if they will open inbound port 25 traffic for you, and you should then be able to relay outbound mail through their mailserver (which isn't a bad idea anyway)?
netstat -nalp | grep master | grep "LISTEN "
should tell you what postfix is listening on
To make postfix listen on a different port, a line like
IP_address:port inet n - n - - smtpd
in place of
smtp inet n - n - - smtpd
in master.cf will work. You may be able to simply replace smtp with the port number in the second version (without the IP), but I've never tried it.
Simply sending on a different port will achieve nothing, since other servers won't be listening. if you want to relay through a specific server, you can use