Delayed Message: Message features not supported by receiving system
The email message has not yet been delivered because the message seems to contain features not supported by the receiving system. Typically, this occurs when systems have differing capabilities.
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.
4.3.3: 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.
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.