Another way is, make Postfix listen on addition port, e.g. port 2525, then configure your application to use this port instead. Steps:
*) Open /etc/postfix/master.cf, find below line:
smtp inet n - - - - smtpd*) Copy it, but replace the first "smtp" to port number you want to use. for example, port 2525.
smtp inet n - - - - smtpd
2525 inet n - - - - smtpd*) Restart Postfix service, then it will listen on port 25, 587 and 2525.
Note: Please make sure you have port 2525 open in firewall (iptables, pf, etc), so that your application on another server can connect to it.