SMTP response codes: two resources

If you want to learn more about SMTP response codes and error messages, here's a couple of resources you'll want to check out and bookmark for future reference. First, here's John Porrini from SocketLabs: 21 SMTP Response Codes That You Need To Know.

And after you've checked that out, you'll want to bookmark the SMTP FIELD MANUAL: A collection of raw SMTP error codes spotted in the wild from Postmark. Very useful to try to understand what kind of rejects (bounces) certain mailbox providers will send back and sometimes it has come in handy when I can't access to a client's actual bounce message, so I can review what common ones that particular mailbox provider sends, and develop a thoery of what might have happened, based on that.

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