5.6.0

Message Not Delivered: unknown issue with the message content

The message could not be delivered for unknown reasons related to its content or attachments.

Fixes

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.

5.6.0: Official Definition

5.X.X

[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.

X.6.0

[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.6.0:

Something about the content of a message caused it to be considered undeliverable and the problem cannot be well expressed with any of the other provided detail codes.