SMTP – DATA: 500 Syntax error, command unrecognized

After sending a HTML e-mail to a mailing list of about 1500 recipients I am getting a few errors I can’t seem to solve.

The list is being delivered successfully to about 1400 recipients and there are no issues with the major e-mail providers. The errors are all from privately hosted domains.

The first one is:

500 Syntax error, command unrecognized

This happens on the DATA command to about 50 recipients. The second error is actually an empty response on the DATA command that affects about 30 recipients.

To get to the DATA command usually this would mean the address is ok and there are messages available if this is not the case. I’m pretty sure there is nothing wrong with the syntax of the e-mail especially as so many other recipients don’t report such an error.

Is this a case of bad reporting on behalf of the recipient, or is there something that can be done on our end?

The e-mail has the correct line lengths, the multi-part-boundaries are correct and the DATA command is ended with CRLF.CRLF. I’m not really aware of anything else I could do improve the syntax, below is the general format of what is being sent:

Return-path: <>
Subject: Subject 'Test' e-mail' test "5"
Message-Id: <>
Date: Thu, 9 Jul 2020 10:52:00 +0000
To: <>
From: Somwhere <>
Reply-To: <>
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="multipart-boundary"

Content-Type: text/plain; encoding="UTF-8"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

This is a HTML e-mail


Content-Type: text/html; encoding="UTF-8"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

body {

<table height="100%" width="100%"  cellpadding="0" cellspacing="0" border="0">