Common Error Codes
421 FTP response code
A 421 response code indicates that while the service is still running, the service is unavailable at the time of connection. It indicates that the server will be restarting as soon as it finishes processing pending operations (usually any file transfers currently in progress). It is considered a transient negative response, which means the client is encouraged to issue the same command again at a later time when it can be accepted.
421 Service not available, closing control connection.
A 421 FTP response code may be given in response to any FTP command or connection attempt if the FTP server knows it must shut down.