In message <D25FD080201FAC0F622B58D1(_at_)PST(_dot_)JCK(_dot_)COM>, John C
Klensin writes:
--On Friday, September 02, 2011 09:32 +1000 Mark Andrews
<marka(_at_)isc(_dot_)org> wrote:
However, if a mail server is online for only a few hours a
day, then I'd seriously suggest getting a backup mail server
to accept messages during the downtime (or putting the main
mail server somewhere else with a better connection), unless
you don't really care about getting mail...
And poll when coming online using TURN after authenticating.
Also if you do this you should ensure that your backup MX can
reject non valid addresses or you will generate a lot of
backscatter. There are a number of different techniques that
can be used to do this.
That is one of several reasons why TURN was deprecated years ago.
TURN is useful if the is not a direct incoming path.
e.g. Running multiple MTA's behind a firewall/nat that blocks incoming
connections.
ETRN is useful if the is a direct incoming path.
Horses for courses. That said ETRN is probably more applicable
here as there is a direct incoming path.
Backscatter is a problem for any backup MX.
Note the "backup MX" may be the only MX listed to the public and
the backup MX can see lower priority MX records.
Mark
--
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: marka(_at_)isc(_dot_)org