Emails are not delivered to recipients
Emails remain in mail queue at Tools & Settings > Mail Server Settings > Mail Queue tab
The mail log at
/var/log/maillogcontains the following error regarding deferred/rejected email:
450 4.7.1 Client host rejected: cannot find your hostname
hostname postfix/smtp: 3660E81C66: host mx-remote-host.com [203.0.113.3] said: 450 4.7.1 <ip-203-0-113-2.another-host.com>: Helo command rejected: Host not found (in reply to RCPT TO command)
Incorrect or missing PTR record of the sender's mail server.
Contact the server hoster or provider of the IP address in order to correct PTR record and make it pointing to the mail server hostname. For Plesk hosted on Cloud server, follow the article below: