Follow the below pasted steps will help you to open An alternate SMTP port(26) on Plesk server.
1. First create a new entry for the alternate port in xinetd
# cd /etc/xinetd.d # cp smtp_psa smtp_psa_p26
2. Modify the newly created smtp_psa_p26 file:
# vi smtp_psa_p26
Edit the first line of the file as “service smtp_p26” and save the file.
3. Next, edit the service to include the newly created one.
# vi /etc/services
Add the following entry:
smtp_p26 26/tcp mail smtp_p26 26/udp mail
After editing this, restart xinetd service.
# service xinetd restart
Now, the server should be listening on port 26.
# netstat -lpn|grep 26