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Understanding SMTP Response Numbers

This is a general overview/breakdown of SMTP numbers and their meanings.

First Digit of SMTP Errors: (1XX)
The first generally tells whether the server accepted the command and if it could handle it.
Digit Explanation
1 The server has accepted the command, but does not yet take action. A confirmation message is required. Currently, this is not used.
2 The server has completed the task successfully.
3 The server has understood the request, but requires further information to complete it.
4 The server has encountered a temporary failure. If the command is repeated without any change, it might be completed. Mail servers can use such temporary failures to keep untrusted senders at bay.
5 The server has encountered an error.
Second Digit of SMTP Errors: (X1X)
The second number gives more information.
Digit Explanation
0 A syntax error has occurred.
1 Indicates a informational reply, for example to a HELP request.
2 Refers to the connection status.
3 Unspecified
4 Unspecified
5 Refers to the status of the mail system as a whole and the mail server in particular.
Third Digit of SMTP Errors: (XX1)
The third number gives a more specific description to the error and can be referenced from Email / SMTP Response and Error Numbers
email/troubleshooting/understanding-smtp-numbers.txt · Last modified: 2015/02/01 10:09 by dlevey