5.7.11: 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.7.11:
This response to the AUTH command indicates that the selected authentication mechanism may only be used when the underlying SMTP connection is encrypted. Note that this response code is documented here for historical purposes only. Modern implementations SHOULD NOT advertise mechanisms that are not permitted due to lack of encryption, unless an encryption layer of sufficient strength is currently being employed.