postfix got "550 Access denied - Invalid HELO name" from relay host

Solution In Progress - Updated -

Issue

  • postfix output the following error in maillog
Sep 14 11:07:00 mail postfix/smtp[5756]: ADFC368204: to=<test.user@abc.example.com>, 
relay=relay.example.com[192.168.100.150], delay=5, status=bounced (host 
relay.example.com[192.168.100.150] said: 550 Access denied - Invalid HELO name (See RFC5321 4.1.1.1) 
(in reply to RCPT TO command))
  • What is the cause of this error?

Environment

Red Hat Enterprise Linux 6

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