Delayed Message: Message priority level too low
The email message has not yet been 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.
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.
4.7.15: 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 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.