[dredmorbius]

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.