A Guide to SMTP Error Codes 

• 211 – System status/system help reply 
• 214 – Help message 
• 220 – Domain service ready 
• 221 – Domain service closing transmission channel 
• 250 – Requested mail action completed and OK 
• 251 – Not Local User, forward email to forward path 
• 252 – Cannot verify user, will attempt delivery later 
• 253 – Pending messages for node started

• 354 – Start mail input; end with . 
• 355 – Octet-offset is the transaction offset 

• 421 – Domain service not available, closing transmission channel 
• 432 – Domain service not available, closing transmission channel 
• 450 – Requested mail action not taken: mailbox unavailable. request refused 
• 451 – Requested action aborted: local error in processing. Request is unable to be processed, try again 
• 452 – Requested action not taken: insufficient system storage 
• 453 – No mail 
• 454 – TLS not available due to a temporary reason. Encryption is required for the requested authentication mechanism 
• 458 – Unable to queue messages for node 
• 459 – Node not allowed: reason 

• 500 – Syntax error, command unrecognized 
• 501 – Syntax error in parameters or arguments 
• 502 – Command not implemented 
• 503 – Bad sequence of commands 
• 504 – Command parameter not implemented 
• 510 – Check the recipient address 
• 512 – Domain can not be found. Unknown host. 
• 515 – Destination mailbox address invalid 
• 517 – Problem with 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 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 

• Check spelling 
• Confirm the mailbox exists 
• Verify DNS MX records 

• Enable SMTP authentication 
• Use the full email address as the username 
• Confirm the correct password

• Wait and retry 
• Check the server status 
• Confirm that no firewall is blocking 

• Ask the recipient to clear the mailbox 
• Reduce attachment size 

• Check SPF/DKIM records 
• Review sending limits 
• Run blacklist check 

5.1.1 → Bad destination mailbox 
5.7.1 → Delivery not authorised 
4.4.1 → Temporary routing issue 

First number = success, temporary, or permanent 
Second number = category 
Third number = specific reason 

If the code starts with: 

4 → Wait or retry 
5 → Investigate and fix 

• X.1.0 – Other address status 
• X.1.1 – Bad destination mailbox address 
• X.1.2 – Bad destination system address 
• X.1.3 – Bad destination mailbox address syntax 
• X.1.4 – Destination mailbox address ambiguous 
• X.1.5 – Destination mailbox address valid 
• X.1.6 – Mailbox has moved 
• X.1.7 – Bad sender’s mailbox address syntax 
• X.1.8 – Bad sender’s system address 

• X.2.0 – Other or undefined mailbox status 
• X.2.1 – Mailbox disabled, not accepting messages 
• X.2.2 – Mailbox full 
• X.2.3 – Message length exceeds administrative limit 
• X.2.4 – Mailing list expansion issue

• X.3.0 – Other or undefined mail system status 
• X.3.1 – Mail system full 
• X.3.2 – System not accepting network messages 
• X.3.3 – System not capable of selected features 
• X.3.4 – Message too big for system 
• X.3.5 – System incorrectly configured

• X.4.0 – Other or undefined network or routing status 
• X.4.1 – No answer from host 
• X.4.2 – Bad connection 
• X.4.3 – Routing server failure 
• X.4.4 – Unable to route 
• X.4.5 – Network congestion 
• X.4.6 – Routing loop detected 
• X.4.7 – Delivery time expired

•   X.5.0 – Other or undefined protocol status 
• X.5.1 – Invalid command 
• X.5.2 – Syntax error 
• X.5.3 – Too many recipients 
• X.5.4 – Invalid command arguments 
• X.5.5 – Wrong protocol version 

•   X.6.0 – Other or undefined media error 
• X.6.1 – Media not supported 
• X.6.2 – Conversion required and prohibited 
• X.6.3 – Conversion required but not supported 
• X.6.4 – Conversion with loss performed 
• X.6.5 – Conversion failed 

• X.7.0 – Other or undefined security status 
• X.7.1 – Delivery not authorized, message refused 
• X.7.2 – Mailing list expansion prohibited 
• X.7.3 – Security conversion required but not possible 
• X.7.4 – Security features not supported 
• X.7.5 – Cryptographic failure 
• X.7.6 – Cryptographic algorithm not supported 
• X.7.7 – Message integrity failure 

Security errors (x.7.x) often relate to: 

• SPF failures 
• DKIM failures 
• DMARC policy rejection 
• IP reputation 

No, SMTP errors relate only to email delivery. 

Yes, email configuration changes can be reverted. 

No, they are temporary and usually retry automatically. 

Not necessarily, many are simple configuration issues.