Bad Command Sequence
The command was issued in an incorrect sequence. For example, RNTO was sent without a preceding RNFR.
503
503
Service Unavailable
The server is not ready to handle the request, typically due to temporary overload or scheduled maintenance.
command-invalid
The client sent a command that is not valid in the current channel or connection state, such as sending a channel-level command before the channel is open. This is a hard error.
Service Unavailable
The server is temporarily unable to handle the request due to overloading or maintenance. The response may include a Retry-After header indicating when the service may be available again.
Bad sequence of commands
The command was issued in an incorrect order. For example, DATA before MAIL FROM, or RCPT TO before MAIL FROM.