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The first number is the email message status. For example, 5.2.1 is a Permanent Error where the receiving email is not accepting messages. The Extended SMTP error codes work in 3 digit combinations. 553 – Requested action not taken: mailbox name not allowed.552 – Requested mail action aborted: exceeded storage allocation.551 – User not local please try forward path.550 – Requested action not taken: mailbox unavailable.538 – Encryption required for requested authentication mechanism.534 – Authentication mechanism is too weak.533 – Remote server has insufficient disk space to hold email.522 – Recipient has exceeded mailbox limit.517 – Problem with senders mail attribute, check properties.515 – Destination mailbox address invalid.504 – Command parameter not implemented.501 – Syntax error in parameters or arguments.
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500 – Syntax error, command unrecognized.458 – Unable to queue messages for node.Encryption required for requested authentication mechanism 454 – TLS not available due to temporary reason.452 – Requested action not taken: insufficient system storage.451 – Requested action aborted: local error in processing Request is unable to be processed, try again.450 – Requested mail action not taken: mailbox unavailable.432 – Domain service not available, closing transmission channel.421 – Domain service not available, closing transmission channel.355 – Octet-offset is the transaction offset.253 – Pending messages for node started.252 – Cannot Verify user, will attempt delivery later.251 – Not Local User, forward email to forward path.250 – Requested mail action completed and OK.221 – Domain service closing transmission channel.211 – System status / system help reply.This list of codes is the standard SMTP error codes you will find in most email bounces. Below is a list of the Standard SMTP error codes and the Extended SMTP (ESMTP) error codes. 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. It is important to figure out why your email bounces.