qconfirm-return expects QSBMF (qmail-send bounce message format) bounce messages and
processes them. It tries to extract the recipient address that caused the
bounce from the failure paragraph and the unique key that
has created for delivery confirmation for messages from this address from
the original message (the included
confirmation request message).
qconfirm-return fails to do so, or the key in the original message does not match the key
has created for this address, or the envelope sender is not empty, it prints
a warning and exits 0.
qconfirm-return notices that this address bounces. If
qconfirm-return sees that this address already has bounced before, it tells
to silently drop the message from this sender address the next time qmail
tries to deliver it (exception: see the -n option).
qconfirm-return was able to successfully parse and notice the bounce message, it exits 99,
and so tells
to ignore all further delivery instructions.
qconfirm-return exits 111 on error.