SMTP is responsible for delivering your email campaigns. If you send bulk emails, you have probably faced the problem of various SMTP errors that prevented your subscribers from receiving from you. Mind that not every reply means an error, as it can be a response to a command including important information about the server. Codes of SMTP errors alone may seem to you incomprehensible digits. To understand what is wrong, knowing the explanations of these codes is essential.

Error codes are made up of three digits and each of them has its own meaning. The first one shows whether your command was accepted and handled.

There are 5 values for the first digit in the code

  1. A command was accepted by the server but no actions have been taken. A confirmation message is required.
  2. The task was completed successfully by the server without any mistakes.
  3. The request was understood by the server but further information is required for completing it.
  4. A temporary failure occurred. If there are no changes while repeating the command — try again.
  5. The server faces a critical error and your command can't be handled.

If an error starts from 2 — your command is successfully handled, from 4 — there is a temporary failure, from 5 — there is a permanent failure. Your emails will not be sent unless you eliminate the failures.

There are 4 values for the second digit in the code

  1. There is a syntax error.
  2. Information reply (Help request, for example).
  3. Shows the connection status.
  4. Shows the status of a mail server or a mail system.

The last digit means more detailed information on the delivery status.

Most important SMTP errors codes:

214 — Reply to the "Help". Information on how to work with a server, description of specific commands, needed by humans only.

220 — Service is running/ready.

221 — The service is closing the transmission channel.

250 — Requested mail action completed and OK. 

251 — The recipient is not local but the server will forward a message.

252 — The user can't be verified by the server but anyway it will try to deliver the email.

354 — Start mail input end with <CRLF>.<CRLF>. The server received the information about the sender and the recipient, and asks for the "Message Body".

421 — The service is not available, the transmission channel is closed.

422 — The size of the email exceeds the limits of the recipient's inbox.

442 — The mail server started to deliver the email but the connection was dropped during the transmission.

450 — The requested action was not taken, the mailbox was unavailable. It may be temporary busy.

451 — The requested action aborted, Local error in processing. 

452 — The requested action not taken – Insufficient storage.

500 — A syntax error because the server could not recognize a command.

501 — A syntax error in arguments or parameters. It can be invalid email address.

502 — The command is not implemented. It is valid but has not been activated by your provider.

503 — Bad sequence of commands, the mail server requires authentication.

504 — A command parameter is not implemented.

510 — Bad email address. Your recipients' emails may contain typos.

512 — Unknown host, domain can't be found.

513 — Address type is incorrect. It can be a misspelling, or the server requires an authentication.

515 — Invalid mailbox address.

521 — Mailbox does not accept emails.

523 — Email's length exceeds the recipient's limits.

530 — Authentication is required, or the recipient's server has blacklisted yours one, or invalid email address.

531 — Mail system full.

534 — Authentication mechanism is too weak, a message is too big.

535 — Authentication errors.

541 — The recipient's address rejected your message. It can be caused by spam-filter.

550 — The requested actions not taken because the mailbox is unavailable. The command could be rejected by policy reasons, or the mailbox is not found, or there is no access. 

551 — User is not local or invalid address.

552 — The action was aborted due to exceeded storage allocation.

553 — Requested action not taken, Mailbox name invalid. 

554 — Transaction failed. It happens when the recipient's server considers your email to be spam or your IP address to be blacklisted.

555 — The email address format was not recognized by the server.

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