Outgoing mail is rejected on Plesk server: Helo command rejected: ACCESS DENIED

Symptoms

• Outgoing mail does not reach the recipient;

• 
/var/log/maillog
has the following error for all the outgoing mail messages:

  <203-0-113-2.example.com>: Helo command rejected: ACCESS DENIED. Your
  email was rejected because the sending mail server appears to be on a
  dynamic IP address that should not be doing direct mail delivery
  (203-0-113-2) (in reply to RCPT TO command)

  ---

  550 Access denied - Invalid HELO name (See RFC2821 4.1.1.1) (in reply to MAIL FROM command))

• Postfix
myhostname
entry has an IP address in it:

  # postconf | grep ^myhostname
  myhostname = ip-203-0-113-2.example.com

• Server hostname has IP address in it:

  # cat /etc/hostname
  ip-203-0-113-2.example.com

Cause

Incorrect hostname.

Resolution

1. Log into Plesk.

2. Change the entry in Tools & Settings > Server Settings > Full hostname to the one without an IP address for example
example.com
.