Cannot send emails: 450 4.7.1 Client host rejected: cannot find your hostname

Symptoms

• 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/maillog contains the following error regarding deferred/rejected email:

  450 4.7.1 Client host rejected: cannot find your hostname

  hostname postfix/smtp[15775]: 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)

Cause

Incorrect or missing PTR record of the sender's mail server.

Resolution