Much nicer behaviour of Courier would be when upon receiving mail from
misconfigured domain the courier itself would send mail to either
Postmaster@thatdomain or dns admin from that domain explaining what's
wrong with their MX record with some clues how to fix it. IMHO many
'admins' would accept it and fix it, because they don't have it
misconfigured on purpose.
How's that possible if their DNS is broken and they can't receive mail?
What do you do with the bounces? Accepting mail from
non-existent/non-routable domains can really come back and bite you in the