I just realised that you may not get the other side of the scenario. On a 5xx series response, you bounce. Where does the bounce go? Back to the original sen

I just realised that you may not get the other side of the scenario. On a 5xx series response, you bounce. Where does the bounce go? Back to the original sender. This is the 'closing of the loop'. Original sender sees the message was not sent successfully. They can now retry, resend, make a phone call, whatever. "Hey Bob, what was your email address again? I got a bounce. Ohhh, damn, I typed bod@ instead of bob@. I'll fix and resend. That's how it works. That's how it works for: - a typo in the email address - when an account is deleted (and maybe your friend has a new email address) - the remote mail admin did something horribly, horribly wrong, and it bounced as a result (Horribly, horribly wrong being -- they told their own mail servers that accounts did not exist, who helpfully passed that info on.)