Command Parameter Not Implemented
The command is implemented but the specific parameter value supplied is not supported by this server.
504
504
Gateway Timeout
The server, while acting as a gateway or proxy, did not receive a timely response from an upstream server.
channel-error
The client attempted a channel-level operation in a situation where it is not appropriate, such as re-opening a channel that has already been closed with a hard error. This is a hard error.
Server Time-out
The server did not receive a timely response from an external server it needed to contact to process the request. Differs from 408 in that this is a failure of a downstream server, not the client.
Command parameter not implemented
A parameter of the command is not implemented by the server, for example an unsupported AUTH mechanism.