Lookup for "451 5.5.6"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (451)
Requested action aborted: local error in processing
Enhanced Status Code Details (5.5.6)
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.