Emails are being rejected by DDEI with the following entry in mail log:
proxy-reject: END-OF-MESSAGE: 501 Error: attribute value too long; from=**FROM-ADDRESS** to=**TO-ADDRESS** proto=ESMTP helo=**MTA-HOSTNAME**
This issue is caused by an email not conforming to RFC:2821 by exceeding the maximum line length:
text line The maximum total length of a text line including the is 1000 characters (not counting the leading dot duplicated for transparency). This number may be increased by the use of SMTP Service Extensions.
This issue should be investigated on the upstream MTA, why it is sending non-RFC compliant emails to DDEI.