When you send mail from Outlook in Exchange Online, you receive a non-delivery report (NDR) that resembles the following:
550 5.0.350 Remote server returned an error -> 550 x-dg-ref header is too long.
This issue occurs if you use Rich Text formatting in Outlook and the message has at least one attachment. The attachment may also be nested so that it contains another attached message.
To resolve this issue so that the message can be delivered successfully, select the HTML option instead of Rich Text Format when you compose email messages.
The HTML option prevents the attachment from being part of the Transport Neutral Encapsulation Format (TNEF) header.
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