For one reason or another some people like to run their SMTP service on a port other than the standard TCP port 25. Whether it's to bypass their ISP's firewall, provide a second instance of SMTP, or maybe just to be different.

This article will show you a simple way to do this on a Linux server running the Plesk control panel.

In '/etc/xinetd.d/' there is a file called smtp_psa, you will need to copy this file to another filename (in this example I'll use smtp2_psa).

In this new file, you will want to change the line:
'service smtp' to 'service smtp2'

Once this is complete, you will need to modify your /etc/services file, and add the smtp2 service:
Simply copy/paste the existing smtp line and change the name and port (we'll use port 26 in this example).

smtp2 26/tcp mail
smtp2 26/udp mail

tcp 0 0 0.0.0.0:26 0.0.0.0:* LISTEN 2345/xinetd

Trying 123.123.123.123...
Connected to hostname.domain (123.123.123.123).
Escape character is '^]'.
220 hostname.domain.tld ESMTP