x2z (Connections): These are replies referring to the transmission channel.
x1z (Information): These are replies to requests for information.
x0z (Syntax): These replies refer to syntax errors, syntactically correct commands that do not fit any functional category, and unimplemented or superfluous commands.
The second digit encodes responses in specific categories:
The SMTP client SHOULD NOT repeat the exact request (in the same sequence).
5yz (Permanent Negative Completion Reply): The command was not accepted and the requested action did not occur.
However, the error condition is temporary, and the action may be requested again.
4yz (Transient Negative Completion Reply): The command was not accepted, and the requested action did not occur.
3yz (Positive Intermediate Reply): The command has been accepted, but the requested action is being held in abeyance, pending receipt of further information.
2yz (Positive Completion Reply): The requested action has been successfully completed.
The first digit denotes whether the response is good, bad, or incomplete.: The number is for use by automata (e.g., email clients) to determine what state to enter next the text ("Text Part") is for the human user.
3.4 - 5yz Permanent negative completionĪ "Basic Status Code" SMTP reply consists of a three digit number (transmitted as three numeric characters) followed by some text.