Message Not Delivered: Message features not supported
The message could not be delivered because the message you are sending seems to contain features not supported by the receiving system. Typically, this occurs when systems have differing capabilities.
Fixes
Contact the sender's email system support team to investigate what feature of your email system may not be supported by the receiving system. This usually requires some level of support to fix.
Also, consider contacting the recipient through an alternative email address or other means.
5.3.3: 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.3.3:
Selected features specified for the message are not supported by the destination system. This can occur in gateways when features from one domain cannot be mapped onto the supported feature in another.