Delayed Message: Message content or attachment not supported
The email message has not yet been delivered because it included content or attachments that the recipient's email provider cannot accept.
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.
4.6.1: 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.
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.