SymptomsAccording to the server-wide mail configuration in Parallels Plesk Panel (PP), web interface authorization is required for mail relay. You may check it here:
For PP 10: Go to Tools & Settings -> Mail Server Settings -> Relay options
For PP 9: Go to Home -> Mail Server Settings -> Relay options
For PP 8: Go to Server -> Mail -> Preferences -> Relay options
However, the server is working as an open relay. The QMail queue is filled with millions of spam messages, and this is preventing PP clients from being able to use the email service.
What might be causing the problem?
CauseThere are several possible causes. One of them is that the Sendmail configuration file exists in /etc/xinetd.d and is overriding the /etc/xinetd.d/smtp_psa settings.
~# grep -lr smtp /etc/xinetd.d/
ResolutionFirst, it is recommended that you check if your server is indeed acting as an open relay. Investigate this and other possible causes of the issue as described in this article:
1394 How do I verify that my server is not acting as an open relay?
If these actions do not resolve the issue, remove the sendmail configuration file from /etc/xinetd.d and restart the extended Internet services daemon (xinetd):
# rm -f /etc/xinetd.d/sendmail
# /etc/init.d/xinetd restart
Stopping xinetd: [ OK ]
Starting xinetd: [ OK ]