Yeah, this is a particularly pernicious failure given how email works. Many mailing providers will just mark these as blacklisted, now, and lots of unsophisticated users likely won't notice.
I consider myself sophisticated enough, but my Bitwarden has 700 accounts, of which ~30% old ones are registered with a gmail address, and the rest are handled behind g suite. Granted that last bit might be partly my fault, even though I paid for it. But even for a "sophisticated" user, I have no easy way of knowing if any of these accounts have silently failed to function now, other than by the passage of time and eventually finding out.
Oh, absolutely, even for sophisticated users mitigating may be difficult or impossible depending on exactly what bounced and how. But you at least are aware that this happened, and that you have a problem. Think how many people are out there with no clue what this error meant, or that it signaled an ecosystem problem, or that just had hundreds or thousands of emails silently bounce and unsubscribe.
