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How to resolve “550 Access denied – Invalid HELO name” error from Outlook (0x800ccc78), Windows Mail, etc in InterWorx Postfix mail servers

The mail user reports the error:

Task user@domain.com – Sending reported error (0x800CCC78): ‘Cannot send the 
message. Verify the email address in your account properties. The server 
responded: 550 Access denied – Invalid HELO name (See RFC2821 4.1.1.1)

Corresponding entry in Postfix logs say:

2014-09-12 12:19:45 H=isp-ip-rev-dns.net [xxx.xxx.xxx.xxx] rejected MAIL : 
Access denied - Invalid HELO name (See RFC2821 4.1.1.1)

At Bobcares help desk support services, this is a common error we see in InterWorx shared hosting servers. Strict anti-spam rules or lack of SMTP authentication usually cause this error. Given below are a few scenarios we’ve encountered, and the solutions we provided for them. (more…)