Message Not Delivered: Message priority level too low
The message could not be delivered because its priority level (as specified by the sender's email program) was lower than is currently acceptable to one of the email systems required to deliver it.
Fixes
In the sender's email program's message composition area, look for a "priority" option. Try looking in the app's settings or options. Try sending the message again with its priority set higher.
If this error persists even when you set the message's priority as high as it can go, contact the email provider for help.
5.7.15: 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.
[RFC6710] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.7.15:
The specified priority level is below the lowest priority acceptable for the receiving SMTP server. This condition might be temporary, for example the server is operating in a mode where only higher priority messages are accepted for transfer and delivery, while lower priority messages are rejected.