But then why would Google's mailserver not know that it once delivered email to that mailbox?
If the protocol is stateful, why the state should be kept by the "sender" and not by the "receiver"? Being stateless removes this ambiguity in my opinion.
Also we should remember how bad is for spam reputation sending emails to a non-existent address and thus I would not blame it on the mailing list for being "overly cautious".
The situation here is that the service was so borked that it didn't know what it didn't know.
Hard-failing good addresses is a much worse bad than soft-failing bad addresses. In the latter case, remote sender tries again later and eventually gets a hard bounce. In the former, good addresses are permanently dropped from numerous services, and sent mail is lost rather than retried.
Critical failures should soft bounce until positively determined otherwise.
If the protocol is stateful, why the state should be kept by the "sender" and not by the "receiver"? Being stateless removes this ambiguity in my opinion.
Also we should remember how bad is for spam reputation sending emails to a non-existent address and thus I would not blame it on the mailing list for being "overly cautious".