Lookup for "501 4.7.11"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (501)
Syntax error in parameters or arguments
Enhanced Status Code Details (4.7.11)
Encryption required for requested authentication mechanism
The selected authentication mechanism may only be used when the underlying SMTP connection is encrypted. This response code is documented 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.