Lookup for "503 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 (503)
Bad sequence of commands
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.