421 FTP Reply Code - KB Article #1519Related Articles --
421 Service not available, closing control connection.
A 421 reply code indicates that while the service is still running, the service is unavailable at the time of connection. It usually indicates that the server will be restarting as soon as it finishes processing of 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 may be accepted.
A 421 reply code may be given in response to any FTP command or connection attempt if the FTP server knows it must shut down.