I have 2 responses to the sender reputation concern:

1. If the user's mail service penalizes you equally regardless of whether the recipient's addressed existed 1 day vs. never existed, that itself is absolutely inexcusable nonsensical behavior that needs to be fixed. You shouldn't do that, just as you shouldn't shoot the mailman (or even arm yourself...) merely because he knocked a second time.

2. Notwithstanding the previous point, I don't buy this as valid justification anyway. The proposal isn't that you should blast 100 emails toward the mailbox every time you get a bounce due to an address not existing. The idea was to just exercise some intelligence in the matter. Like maybe just retry a couple times, spaced out by a day or two. The bounce rate increase due to such an event is very negligible here—people don't suddenly delete their accounts en masse. When that happens, it's clearly due to an outage, not because half the users at that domain suddenly decided to delete their accounts. (Which is something you can also easily detect across the domain as another useful signal to drastically lower the bounce rate across the entire domain, btw, if you're absolutely paranoid about your immaculate delivery rate dropping by an epsilon. But it shouldn't be necessary given how negligible the impact should be.)

So I don't buy this excuse one bit.

> The proposal isn't that you should blast 100 emails toward the mailbox every time you get a bounce due to an address not existing. The idea was to just exercise some intelligence in the matter. Like maybe just retry a couple times, spaced out by a day or two.

What you're proposing is to explicitly ignore the specification (which says that you should _not_ retry when you receive a 550) and try to implement a custom smart retry logic that handles temporary error cases, but also does not get you blocked.

> So I don't buy this excuse one bit.

I'm all for building resilient services, but "try to detect when a server incorrectly returns 550" is not something I would prioritize at all. I'll happily manually clean up after this occurrence than to have this complicated retry logic. It's not an "excuse", it's a very sensible trade-off.

No, I am quite explicitly not ignoring the spec. It quite deliberately says should not, not must not. If anyone is ignoring the spec here, it's you, not me. Should not is sound advice; it's telling you what you're supposed to do when you don't have a reason to behave differently. You know, like how you "should not" leave the lights on when you leave your room. Or—more pertinently here—how you "should not" assume everyone is a liar. But when you actively see evidence that deviates from the norm, you are given the power—and arguably the responsibility—to exercise your discretion here to adapt to the situation. If the spec wanted blind obedience, it would say "must not" like it did in 60 other places, but it quite obviously and intentionally decided that would be unwise, and this scenario seems like a pretty clear illustration of that.

But the RFC isn't only for senders it's also for receivers, isn't it?

That means there are two sides to the interpretation of what SHOULD NOT means. And in this case, senders have, due to experience, interpreted what Google does when someone SHOULD NOTs:

- The sender SHOULD NOT send us the same sequence again when we reply 550, if they do they MUST go on our shitlist.

Obviously it's not so binary and it takes retrying to several different recipients, but people have very good reason to interpret this SHOULD NOT as MUST NOT.

No, that's not a sane way to interpret this RFC for the receiver either. I already answered this, so you'll have to go back to my earlier comment (this might be my last comment as I won't keep repeating myself): any system (be it Google's or anyone else's) that penalizes you equally regardless of whether the recipient's addressed existed 1 day vs. never existed is just plain trash. A sender that attempts delivery to an address that accepted their email a day ago is obviously unlikely to be a spammer; there's no justification for treating them as one. It is absolutely unreasonable to interpret the sentence this way. Just as it's unreasonable to interpret "the mailman shouldn't knock a second time when he's told the recipient has moved" as "I should never open the door for the mailman ever again if he does so".