You can do this using 2 methods.
1) Login to Plesk >> Mail Server Settings >> Check “Enable message submission”.
Click OK.
Open that port 587 in csf or apf firewall.
You should now be able to send mail through your server on port 587
2) Make a copy of “smtp_psa” and rename that to “smtp_alternate”
cp -ar /etc/xinetd.d/smtp_psa /etc/xinetd.d/smtp_alternate
Insert the following on top :
=============
service = smtp_alternate (Should be the same name as the file created)
type = UNLISTED
port = 2525
=============
So that the file should look like :
[root @ a24uall ] # cat /etc/xinetd.d/smtp_alternate
service smtp
{
service = smtp_alternate
type = UNLISTED
port = 2525
socket_type = stream
protocol = tcp
wait = no
disable = no
user = root
instances = UNLIMITED
env = SMTPAUTH=1
server = /var/qmail/bin/tcp-env
server_args = -Rt0 /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
}