As you examine a NDR message, look out for a three-digit code, for example, 5.2.1. If the first number begins with 5.y.z, then it means you are dealing with a permanent error; this message will never be delivered. Occasionally, you get NDRs beginning with 4.y.z, in which case there is hope that email will eventually get through. The place to look for the NDR status code is on the very last line of the report.
NDR codes like 5.5.0 or 4.3.1 may remind you of SMTP errors 550 and 431. Indeed, the 500 series in SMTP has a similar meaning to the 5.y.z codes in an NDR - failure. I expect that you have worked out why there are no 2.y.z NDRs? The reason is that the 2.y.z series mean success, whereas Non-Delivery Reports, by definition, are failures.
NDR Classification
As you are beginning to appreciate, these status codes are not random. The first number 4.y.z, or 5.y.z refers to the class of code, for example, 5.y.z is permanent error. Incidentally, I have not seen any status codes beginning with 1.y.z, 3.y.z, or indeed any numbers greater than 5.7.z.
The second number x.1.z means subject. This second digit, 1 in the previous example, gives generic information where as the third digit (z) gives detail. Unfortunately, I have not cracked the complete code for the second digit. However, I have discovered a few useful patterns, for instance, 5.1.x indicates a problem with the email address, as opposed to server or connector problem. In addition, 5.2.x means that the email is too big, somewhere there is a message limit.
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