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Understanding SMTP Response Numbers

Category: Getting Started

Earlier, we have gone through the Email / SMTP Response and Error Numbers article, in which we have mentioned a list of SMTP errors/responses with their possible resolution. Going through the SMTP errors/responses, you might face a problem understanding the numeric SMTP Error (SMTP ERROR 451, SMTP ERROR 552, etc.). This article will provide you with in-detail instructions on understanding the numeric SMTP Errors/Responses on your own. From now on, you don’t need an expert to get a clear idea of what went wrong with your failed email or so. Let’s learn to understand SMTP Response Numbers.

Standard SMTP errors/responses consist of 3 digits (Example: 451, 552). Every Digit has a number between 1 to 5, and each number represents meaning with respect to its position. Below, we will go through each Digit and will explain the meaning,

First Digit of SMTP Errors: (1XX)

The first generally tell whether the server accepted the command and if it could handle it.

1The server has accepted the command but does not yet take action. A confirmation message is required. Currently, this is not used.
2The server has completed the task successfully.
3The server has understood the request but requires further information to complete it.
4The 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.
5The server has encountered an error.

Second Digit of SMTP Errors: (X1X)

The second number gives more information.

0A syntax error has occurred.
1Indicates an informational reply, for example to a HELP request.
2Refers to the connection status.
5Refers 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 of the error and can be referenced from Email / SMTP Response and Error Numbers.

Example of SMTP Errors/Responses

Following SMTP Errors/Responses ranges between 500-504 most caused due to a broken email client or doesn’t delivery the email due to one of the following reasons,

500The server could not recognize the command due to a syntax error.
501The server could not recognize the command due to a syntax error.
501A syntax error was encountered in command arguments.
502This command is not implemented.
503The server has encountered a bad sequence of commands.
504A command parameter is not implemented.