Lookup for "500 2.5.6"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (500)
Syntax error, command unrecognized [This may include errors such as command line too long]
Enhanced Status Code Details (2.5.6)
Authentication Exchange line is too long
Authentication failed due to the client sending a [BASE64] response that is longer than the maximum buffer size available for the currently selected SASL mechanism.