I have found lists that describe codes 500-504 and 550-554, but I see all sorts of codes these days like 517, 521, etc., that don't have canonical descriptions that I could find. I'd like to know what is the consensus on these result codes and all others in the 500-599 range that are currently in use.
The consensus are that all 5xx errors are permanent fatal delivery failures, and that all the sending mail server needs to know is that the original message is to be returned as undeliverable.
The actual numeric codes are for diagnostic purposes only; and carry no special meaning on their own. Meaning that if you want to know precisely why a message was rejected, tell the recipient server's admin what the error code was and he'll look it up and tell you what happened.
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