Message Not Delivered: Message content or attachment not supported
The message could not be delivered because it included content or attachments that the recipient's email provider cannot accept.
Fixes
If the message had attachments, try sending each attachment in a separate email. Often things like Calendar Invitations or Meeting Reminders can have formats that cause problems. See if you can make an adjustment and resend the message.
If this error persists, contact the email provider for help.
5.6.1: 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.6.1:
The media of the message is not supported by either the delivery protocol or the next system in the forwarding path. This is useful only as a permanent error.