What are the most common error messages that bounce back to you in email ?

What is Email Bounce back?

Email bounceback is email delivery fails, so the email service provider informs the sender that his message could not be successfully delivered to the desired recipient due to an error.

The two main common email bounce pack errors are categories Deferrals (400) or a Fatal Bounce (500)

Error code 400 is delay, so the mail server will try again to send the message.
Error code 500 is a message that was not delivered due to an error.

A list of these codes are standard SMTP Error Codes for email bounces.

101 — The server is unable to connect. (connection port error or server name incorrectly)
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)
220 — The server is ready.
221 — Server Closing connection in transmission channel
250 — Requested mail action okay completed (Email Will be delivered)
251 — User not local will forward path (transfer action)
252 — Cannot verify the user, but it will try to deliver later
253 — Pending messages for server started


354 — “Start mail input end <CRLF>.<CRLF>”) its response to the DATA
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 incoming server.
449 - Routing error.
450 – Requested mail action not taken: mailbox unavailable. request refused (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 temporary reason. Encryption required for requested authentication mechanism
458 – Unable to queue messages for node
459 – Node not allowed: reason
471 - local anti-spam filter Problem.


500 – Syntax error, server couldn’t recognize
501 – Syntax error in parameters or arguments
502 – Command not implemented
503 – Bad sequence of commands (requires an authentication)
504 – Command parameter not implemented
510/511 – Check the recipient address
512 – DNS error, Domain can not be found.
515 – Destination mailbox address invalid
517 – Problem with senders 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 same IP. Required Authentication
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 forward path
552 – Requested mail action aborted: exceeded storage allocation
553 – Requested action not taken: mailbox name not allowed
554 – Transaction failed ,incoming server thinks that your email is spam, or your IP has been blacklisted.

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