Ben wrote on 08/03/2007 08:42:47 PM:

> Non-legitimate Non-subscribers (i.e. spam) will never receive the
> OK/Confirmation, will never approve/confirm, and voila, nearly all spam
will
> be kept out of the list (some always manages to trickle through, but you
will
> have made a big dent).

Which will get you listed on some RBLs for backscatter spam as you send
all those confirm messages to the alleged senders.

Better to filter the input to drop such messages or better yet, return a
failure code during the SMTP connection so legitmate messages bounce back
to the sender and they know it didn't go through.  The RFCs still advise
against simply throwing away mail, but that was written before the spam
problem reached the proportions it has today.