Bouncing vs. freezing

    I found that if someone sends to user@???, the only
    default router fails, and the message gets frozen for the
    reasons unclear to me.

    Is there some "standard" (inexpensive) way to bounce
    everything that didn't match any router back to the sender,
    with no postmaster intervention?

    In general, there might be some more explicit logics
    with freeze-bounce-delay behaviour, no_freeze_on_failure
    or something.

    Thank you,
    Ilya