The big problem, I suspect, is that some big mail services might be
adopting a "silently reject" policy, simply discarding mail they
don't like instead of returning a failure reply. My choir uses a
Yahoo mail account (against my advice), and a problem we had was
that messages to all members went only to some members, and we were
never notified about the failures. I'm no longer on the choir
organising committee, so I don't know whether they are still doing
it.