SMTP and ESMTP Error Code List

It is important to figure out why your email bounces. If you receive an email bounce, you can look in the bounce message to find the proper error code that will give you the reason for the bounce. Below is a list of the Standard SMTP error codes and the Extended SMTP (ESMTP) error codes.

SMTP Error Codes

This list of codes is the standard SMTP error codes you will find in most email bounces.

200 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

300 Codes

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

400 Codes

• 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 temporary reason. Encryption required for requested authentication mechanism
• 458 – Unable to queue messages for node
• 459 – Node not allowed: reason

500 Codes

• 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 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

Extended SMTP (ESMTP) standards

The Extended SMTP error codes work in 3 digit combinations. For example, 5.2.1 is a Permanent Error where the receiving email is not accepting messages. The first number is the email message status. The second 2 numbers pinpoint the exact information bout the error.

• 2 Successfully sent the email;
• 4 Temporary problem when sending the email. The email server typically will try to send it again till it reaches retry timeout.
• 5 Permanent or Fatal error. This can be caused by a non existent email address, DNS problem, or your email was blocked by the receiving server.

Below is a list of the Extended SMTP error codes.

Note! X can be a 4 or a 5 depending on is its Temporary or Permanent.

x.1.x Codes

• 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.x Codes

• 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.x Codes

• 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.x Codes

• 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.x Codes

• 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.x Codes

• 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.x Codes

• 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