This is one of the most simple tasks, yet I had to Google around for hours: change the incoming port of postfix's SMTP, without iptables prerouting.
/etc/postfix/master.cf, and search for the following line (usually the first uncommented line):
smtp inet n - - - - smtpd
To change the port, simply write the number instead of smtp in the begining.
2525 inet n - - - - smtpd
and the system can not accept messages on the port you added. Don't forget to enable the port on your firewall, if you have one!