Re: [Exim] roadrunner broke my new toy..

On Mon, 7 Jun 2004, Jeff Lasman wrote:

> We're looking at using callouts to determine if recipients exist. Is
> this a reasonable use of callouts?

Well, I won't say outright that it isn't, but try this for logic: if
the primary MX is willing to respond to callouts from the backup, why
isn't it accepting the mail in the first place? The backup MX is only
supposed to be used as a backup.

If your considering load-sharing as opposed to backup, then it would
be better for all the MXs to know, on their own authority, what is or
isn't a valid localpart (e.g by mirroring the user data, or using a
common database reference such as LDAP).

> Can it work? Will it throw us any other problems?

If the primary is the only source of knowledge about valid localparts,
what are you going to do when the primary is down? Unless you accept
mail "blind" while waiting for it to come back up, then there was
little point in having a backup MX anyway. (OK, it protects you
against partial loss of network connectivity, but for a properly
organised network that should be a rare occurrence).

Having the backup MX accept mail "blind" while the primary was down
used to be a normal approach, way back; but it's getting increasingly
untenable "thanks" to spammers and viruses which fake sender
addresses: if you *do* accept mail "blind", you'll risk sending
bounces to faked sender addresses when the primary comes back up,
right?

> Our other option would be to maintain a list on the secondary MX;

Indeed

> that would be harder to do, and wouldn't be as close to realtime.

But if the primary is where the list of valid addresses lives,
then when the primary is down, the list of valid addresses isn't
going to be changing much...?

I don't know your exact situation, so these are only some random
thoughts which you might want to consider.