Delayed Message: Message content not accessible to recipient server
The email message has not yet been delivered because it included content or attachments that needed to be converted but that was not completed.
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.
4.6.5: 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.
A conversion was required but was unsuccessful. This may be useful as a permanent or persistent temporary notification.