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SMTP 규약 은 rfc2821 / rfc1123 / rfc1893 / rfc2034 이며

코드만 요약 및 실 커넥션간 발생한 메시지도 넣어보았습니다.

코드에 따른 메시지가 100% 정확하지는 않으며 SMTP서버에 따라 메시지가 다릅니다.

100 코드
- 101 : The server is unable to connect.
- 111 : Connection refused or inability to open an SMTP stream.

200 코드
- 211 : System status message or help reply.
- 214 : A response to the HELP command.
- 220 : The server is ready.
- 221 : The server is closing its transmission channel. It can come with side messages like "Goodbye" or "Closing connection".
- 235 : Authentication Succeeded
- 250 : Its typical side message is "Requested mail action okay completed": meaning that the server has transmitted a message.
- 251 : "User not local will forward": the recipient's account is not on the present server, so it will be relayed to another.
- 252 : The server cannot verify the user, but it will try to deliver the message anyway.

300 코드

- 334 {BASE64 인코딩 문자} : VXNlcm5hbWU6 => "Username:" / UGFzc3dvcmQ6 => "Password:" 로그인 진행간 발생하는 코드, 메시지
- 354 : The side message can be very cryptic ("Start mail input end <CRLF>.<CRLF>"). It's the typical response to the DATA command.

400 코드
- 420 : "Timeout connection problem": there have been issues during the message transfer.
- 421 : The service is unavailable due to a connection problem: it may refer to an exceeded limit of simultaneous connections, or a more general temporary problem.
- 422 : The recipient's mailbox has exceeded its storage limit.
- 431 : Not enough space on the disk, or an "out of memory" condition due to a file overload.
- 432 : Typical side-message: "The recipient's Exchange Server incoming mail queue has been stopped".
- 441 : The recipient's server is not responding.
- 442 : The connection was dropped during the transmission.
- 446 : The maximum hop count was exceeded for the message: an internal loop has occurred.
- 447 : Your outgoing message timed out because of issues concerning the incoming server.
- 449 : A routing error.
- 450 : "Requested action not taken – The user's mailbox is unavailable". The mailbox has been corrupted or placed on an offline server, or your email hasn't been accepted for IP problems or blacklisting.
- 451 : "Requested action aborted – Local error in processing". Your ISP's server or the server that got a first relay from yours has encountered a connection problem.
- 452 : Too many emails sent or too many recipients: more in general, a server storage limit exceeded.
- 471 : An error of your mail server, often due to an issue of the local anti-spam filter.

500 코드
- 500 : A syntax error: the server couldn't recognize the command.
- 501 : Another syntax error, not in the command but in its parameters or arguments.
- 502 : The command is not implemented.
- 503 : The server has encountered a bad sequence of commands, or it requires an authentication.
- 504 : A command parameter is not implemented.
- 510/511 : Bad email address.
- 512 : A DNS error: the host server for the recipient's domain name cannot be found.
- 513 : "Address type is incorrect": another problem concerning address misspelling. In few cases, however, it's related to an authentication issue.
- 523 : The total size of your mailing exceeds the recipient server's limits.
- 530 : Normally, an authentication problem. But sometimes it's about the recipient's server blacklisting yours, or an invalid email address.

- 534 : 5.7.14  https://support.google.com/mail/answer/78754 tq6sm3601979pbc.57 - gsmtp 구글 보안에 막힌경우
- 541 : The recipient address rejected your message: normally, it's an error caused by an anti-spam filter.
- 550 : It usually defines a non-existent email address on the remote side.
- 551 : "User not local or invalid address – Relay denied". Meaning, if both your address and the recipient's are not locally hosted by the server, a relay can be interrupted.
- 552 : "Requested mail actions aborted – Exceeded storage allocation": simply put, the recipient's mailbox has exceeded its limits.
- 553 : "Requested action not taken – Mailbox name invalid". That is, there's an incorrect email address into the recipients line.
- 554 : This means that the transaction has failed. It's a permanent error and the server will not try to send the message again.

SMTP커넥션간 400 부터 499 까지는 에러로 간주하고 재시도, 500부터 599까지는 실패처리 해버리시면 간단합니다.

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