On Mon, 27 Feb 2012 09:11:06 +0000, Martin Brown wrote:

The trouble with blacklisting is that smarthosts for major ISPs can and
do end up on the certain blacklists after a botnet or Trojan affects
some of their users.

But a decent ISP will get their machines off blacklists quickly and
being a proper MTA will retry sending the queued messages or, if the
recipient completely rejects(*) the message, bounce it back to the
sender.

(*) Safer to say "temporary problem, call back later" rather an out
and out rejection, spammers rarely retry, proper MTAs do.

--
Cheers
Dave.