Racker Hacker Words of wisdom from a server administrator

If you can’t send mail via port 25 due to blocks imposed by your ISP, you can enable the submission port within Plesk pretty easily. There’s two methods:

The iptables way:
iptables -t nat -A PREROUTING -p tcp --dport 587 -i eth0 -j REDIRECT --to-ports 25

The xinetd way (recommended):
# cd /etc/xinetd.d
# cp smtp_psa smtp_additional
# vi smtp_additional

Make the first line say “service submission” and save the file. Then restart xinetd:

/etc/rc.d/init.d/xinetd restart

This is no longer needed in Plesk 8.4. To enable the submission port in Plesk 8.4, log into the Plesk interface as the Administrator, click Server and click Mail.