Albert Louw writes:
Although it's very easy to say that a 'off-line' mailhost should not
have an MX record,
Indeed. It's rude to put an IP address into a global DNS record if the
address isn't globally reachable. You're wasting valuable client time
that could have been used for successful deliveries to other hosts.
Most MTAs, under most circumstances, will try at least two MX records,
but if you're relying on this then you should fix your configuration.
1000 recipients, 28.8 modem, 10 seconds. http://pobox.com/~djb/qmail/mini.html