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.
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.