PROBLEM
The message bounced with the following error:
552 5.2.0 Invalid 7bit DATA (in reply to end of DATA command)
The following MIME is used:
MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="MAIL_BOUNDARY" Content-Transfer-Encoding: 7bit
SOLUTION
To resolve the issue, contact your third-party mail editor to modify the "Content-Transfer-Encoding" parameter from:
Content-Type: multipart/mixed; boundary="MAIL_BOUNDARY" Content-Transfer-Encoding: 7bit
to:
Content-Transfer-Encoding: binary