What is an Email Bounceback?
An email bounceback occurs when email delivery fails, prompting the email service provider to notify the sender that their message could not be successfully delivered to the intended recipient due to an error.
There are two main categories of common email bounceback errors: Deferrals (400) and Fatal Bounces (500). Error code 400 indicates a delay, so the mail server will attempt to resend the message. Error code 500 signifies that the message was not delivered due to an error.
Below is a list of standard SMTP error codes for email bounces:
1. Connection and server-related errors:
- 101 — The server is unable to connect (connection port error or incorrect server name).
- 111 — Connection refused or inability to open an SMTP stream (connection issue with the remote SMTP server, firewalls, or misspelled domains).
- 211 — System status / system help reply (server information).
- 214 — HELP command (server FAQ).
2. Transmission and delivery errors:
- 220 — The server is ready.
- 221 — Server closing connection in the transmission channel.
- 250 — Requested mail action okay completed (email will be delivered).
- 251 — User not local; will forward the path (transfer action).
- 252 — Cannot verify the user, but will try to deliver later.
- 253 — Pending messages for server started.
3. Mail input and routing errors:
- 354 — Response to the DATA command ("Start mail input; end <CRLF>.<CRLF>").
- 355 — Octet-offset is the transaction offset.
- 420 — Timeout connection problem (recipient’s firewall or hardware problem).
- 421 — Exceeded limit of simultaneous connections; service is unavailable.
- 422 — Recipient's mailbox storage limit exceeded.
- 431 — Out of memory.
- 432 — Recipient's Exchange Server incoming mail queue stopped.
- 441 — Recipient's server is not responding.
- 442 — Connection was dropped (network connection problem due to router).
- 446 — Maximum hop count was exceeded (internal loop).
- 447 — Outgoing message timed out in the incoming server.
- 449 — Routing error.
- 450 — Requested mail action not taken: mailbox unavailable (mailbox corrupted or placed on an offline server, IP problems, or blacklisting).
- 451 — Requested action aborted: local error in processing request (rejection due to remote antispam filter or sending a large bulk email).
- 452 — Too many emails sent or too many recipients.
- 453 — No mail.
- 454 — TLS not available due to a temporary reason; encryption required for requested authentication mechanism.
- 458 — Unable to queue messages for node.
- 459 — Node not allowed for a specific reason.
- 471 — Local anti-spam filter problem.
4. Syntax, authentication, and mailbox errors:
- 500 — Syntax error; server couldn't recognize the command.
- 501 — Syntax error in parameters or arguments.
- 502 — Command not implemented.
- 503 — Bad sequence of commands (requires authentication).
- 504 — Command parameter not implemented.
- 510/511 — Check the recipient address.
- 512 — DNS error; domain cannot be found.
- 515 — Destination mailbox address invalid.
- 517 — Problem with the sender's mail attribute; check properties.
- 521 — Domain does not accept mail.
- 522 — Recipient has exceeded mailbox limit.
- 523 — Server limit exceeded; message too large.
- 530 — Access denied; authentication required.
- 531 — Mail system full.
-533 — Remote server has insufficient disk space to hold email.
-534 — Authentication mechanism is too weak; message too big.
-535 — Multiple servers using the same IP; authentication required.
-538 — Encryption required for requested authentication mechanism.
-540 — Email address has no DNS server.
-541 — No response from host.
-542 — Bad connection.
-543 — Routing server failure; no available route.
-546 — Email looping.
-547 — Delivery time-out.
-550 — Requested action not taken: mailbox unavailable.
-551 — User not local; please try the forward path.
-552 — Requested mail action aborted: exceeded storage allocation.
-553 — Requested action not taken: mailbox name not allowed.
-554 — Transaction failed; the incoming server considers your email spam, or your IP has been blacklisted.
Understanding these error codes can help you diagnose the cause of a bounced email and take appropriate action to ensure successful delivery in the future. Some common solutions include checking for misspelled email addresses, verifying that the recipient's mailbox is active and not full, and ensuring that your email content or sending practices do not trigger spam filters. In some cases, you may need to contact the recipient or their email service provider to resolve the issue.