What do the numbers in SMTP error codes mean?

EQ Admin

EQ Forum Admin
Staff member
SMTP error codes are generally broken down into two categories of messages.

The first is 4xx smtp error codes. Any error code in the 400's is a temporary error. You can infer from this that your outgoing mail server will continue to try and send the email in the hopes that it can be delivered at a later time.

The second is 5xx smtp error codes. Error codes in the 500's are permanent errors. When a 5xx error is encountered your outgoing mail server should immediately stop trying to send the email and send you a bounce message, usually from mailer-daemon, letting you know that the email could not be delivered.

Both categories of error codes should also provide a specific reason for why the error occurred allowing you to troubleshoot the problem and take the appropriate action going forward.