Emails are not sent from or to a Plesk server: 450 4.7.1 Client host rejected: cannot find your hostname

Symptoms

• Emails are not delivered to email addresses hosted in Plesk or not sent to external hosts. The following error messages appear in /var/log/maillog

  450 4.7.1 Client host rejected: cannot find your hostname

  NOQUEUE: reject: RCPT from unknown[203.0.113.2]: 450 4.7.25 Client host rejected: cannot find your hostname, [203.0.113.2]; from=<sender@example.com>

  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)

• PTR record of the sender's mail server points to a different server instead of the sender's mail server:

  # dig MX example.com +short
  mail.example.com
  
  # dig A mail.example.com +short
  203.0.113.2
  
  # dig -x 203.0.113.2
  ip-203-0-113-2.another-host.com // should be mail.example.com

Cause

The sender's mail server has invalid PTR record (reverse DNS).

Resolution