Delayed Message: Content loss during delivery
The email message has not yet been delivered because it included content or attachments that needed to be converted and information was lost in the process.
contact the recipient(s) to ensure that no undesirable consequences result from information being lost from this message before it was delivered.
If the message had attachments, try sending it again without them, and then try sending each attachment in a separate email. If this error occurs when there are no attachments, check the text of the message for unusual content or characters. If it occurs with a specific type of attachment, consider sending that information in another format.
4.6.4: Official Definition
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
This is a warning sent to the sender when message delivery was successfully but when the delivery required a conversion in which some data was lost. This may also be a permanent error if the sender has indicated that conversion with loss is prohibited for the message.