Understanding SMTP Response Numbers
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.
| 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 an 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 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,
| SMTP ERROR/RESPONSE CODE | MEANING |
| 500 | The server could not recognize the command due to a syntax error. |
| 501 | The server could not recognize the command due to a syntax error. |
| 501 | A syntax error was encountered in command arguments. |
| 502 | This command is not implemented. |
| 503 | The server has encountered a bad sequence of commands. |
| 504 | A command parameter is not implemented. |