On 12/08/2010 17:16, John R Levine wrote:
(It's tricky in these cases because it's a 'non-standard' setup -
with a catch-all POP3 account at an ISP, then a mail collector
collecting from there, and sorting messages out and rejecting
messages to unrecognised users - what should happen there? You can't
do an SMTP reject, because the message has already been 'accepted for
final delivery' by the ISP, so you have to send an NDN or not - the
user isn't going to read the messages, so an NDN is appropriate, but
Could you do some sort of technical hack, a call-forward to whatever
is doing the sorting to find out what addresses are valid?
Well, if we were the ISP we could - but we're not... The ISP could be
anyone - we do the end-user mail server software.
(I'm not asking for a solution really - just saying that the 'NDN' issue
isn't that easy - if an upstream ISP blocks outgoing port 25 (blocking
sending using MX routing), and then blocks all outgoing mail because of
'spam' (NDNs) then you have no choice but to not send NDNs).
(NB - We try to encourage users to switch to incoming SMTP if possible
because then an SMTP reject happens, but if they have dynamic IP
addresses it won't work, or if they are are technically incompetent
(many are) then even just opening up a port in a firewall to allow SMTP
in is not possible.)