--On Saturday, May 23, 2009 15:37 +0200 Alessandro Vesely
I've tried and summarized the results of the thread in
There is a third important case although "backup MX" may or may
not be the right terminology for it. In that case, the target
host is basically not attached to the same network as
normal/public Internet-based senders -- the situation may not
involve an outage. Examples:
(1) The target host is on Mars and can be accessed,
easily and normally, by other Martian hosts. However,
for a host on earth, the target host will rarely be
reachable via a normal TCP/SMTP connection and the
intermediate MX host may have the needed delay-tolerant
(2) Similar situations apply with a server on a firewall
(or equivalent) when the internal hosts still use public
address space. Then one might have
foo IN MX 0 bar.example.org.
IN MX 10 baz.example.net.
where bar uses public address space but is not
accessible from outside the "example.org" enterprise
network and baz is public.
(3) The developing country example you mention may be
part of this case, rather than the "outage" one.
The article also seems to omit the important point that there is
no requirement that a backup MX use SMTP outbound. Once it has
gotten the mail, it may use SMTP to get the mail to the delivery
server, but it may use any other mutually-acceptable transport
mechanism, including some batch or bucket procedure and, e.g.,
CDs in the post.