That documentation says users must employ authenticated SMTP relay services, typically connected via TCP port 587 submission, or alike.
Are you configured this port in your CWP server..??
If no, to you enable port 587, edit the file /etc/postfix/master.cf
nano /etc/postfix/master.cf
and remove the # in front of the line:
#submission inet n - n - - smtpd
so that it looks like this:
submission inet n - n - - smtpd
and restart postfix:
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systemctl reload postfix
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So, you will have a submission authenticated port, to you use it.