Plesk : Configure qmail to use alternate SMTP port 26

You have a plesk dedicated server and having issues related to SMTP port 25. There are issues where ISP blocks SMTP port 25, in this case we can use alternate port (I will use port 26).
Kindly follow the below steps to change SMTP port on your plesk dedicated (linux) server to 26.

Login to your server as root

root# cd /etc/xinetd.d

root# ls -l | grep smtp*

root# cat smtp_psa
service smtp
{
socket_type = stream
protocol = tcp
wait = no
disable = no
user = root
instances = UNLIMITED
server = /var/qmail/bin/tcp-env
server_args = /usr/sbin/rblsmtpd -r bl.spamcop.net /var/qmail/bin/relaylock /var/qmail/bin/qmail-smtpd /var/qmail/bin/smtp_auth /var/qmail/bin/true /var/qmail/bin/cmd5checkpw /var/qmail/bin/true
}

root# nano /etc/services

Add the below lines

smtp_psa_new 26/tcp mail
smtp_psa_new 26/udp mail

root# cp smtp_psa smtp_psa_new

Change the service line in the new file “smtp_psa_new” to be this:

service smtp_psa_new
{
socket_type = stream
protocol = tcp
wait = no
disable = no
user = root
instances = UNLIMITED
server = /var/qmail/bin/tcp-env
server_args = /usr/sbin/rblsmtpd -r bl.spamcop.net /var/qmail/bin/relaylock /var/qmail/bin/qmail-smtpd /var/qmail/bin/smtp_auth /var/qmail/bin/true /var/qmail/bin/cmd5checkpw /var/qmail/bin/true
}

root# /etc/init.d/xinetd restart

And you should see smtp listening on ports 25, and 26:

root# netstat -anp | grep xinetd
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 6989/xinetd
tcp 0 0 0.0.0.0:26 0.0.0.0:* LISTEN 6989/xinetd