Make email bounces your friend. 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