smtp field manual
So you’ve seen a new bounce message and you’re wondering how best to handle it. Where do you start? How do you troubleshoot it? I’d love to write a whole book on the topic, but alas, that’ll take some time. In the meantime, here’s what to do:Here’s the very short and abbreviated version of what you should do:READ THE BOUNCE MESSAGE. It’ll contain text. That text will almost always contain an English language description of why the ISP rejected the email message. That message may be cryptic, but it’s VERY insightful, if you can decode it.Trust the bounce message. Bounce messages should be considered correct until proven otherwise. For example: if you get a bounce message from Gmail that says your mail is bouncing because of a lack of email authentication — lean hard into that. Investigate. Test. Prove that authentication is working. Because it’s probably not. Yes, sometimes an ISP
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.