Arnt Gulbrandsen wrote:
An example, easily done with postfix: A set of good addresses are
(ir/)regularly copied from the primary mx, and the secondary replies 4xx
for other addresses. Done.
If both the primary and secondary MX servers are administered by the
same organization or by closely-cooperating organizations, this works
well. However, there are ISPs that offer "secondary MX service" to
their customers, and they are unlikely (or sometimes unable) to make
special arrangments to synchronize user lists.
The fact is, we will never entirely eliminate the situation in which
an SMTP server has accepted a message only to find out that it can't
be delivered, and we have to have a reasonable set of guidelines
for what to do.
I believe that SMTP servers SHOULD generate NDNs unless there is a
compelling local policy reason not to (for example, message contains a
known virus or message is strongly believed to be spam.)
Regards,
David.