Message Not Delivered: Message content could not be processed for delivery
The message could not be delivered because it included content or attachments that needed to be converted, which the email system was unable to accomplish
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 this error persists, contact the email provider for help.
5.6.3: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.6.3:
The message content must be converted in order to be forwarded but such conversion is not possible or is not practical by a host in the forwarding path. This condition may result when an ESMTP gateway supports 8bit transport but is not able to downgrade the message to 7 bit as required for the next hop.
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