Re: [Exim] Delaying secondary mail exchange

On Thu, 2004-04-29 at 22:54 +0100, Tim Jackson wrote:
> b) find a provider who will co-operate in providing a backup MX with
> strong spam-fighting policies and (at least) callouts to your machine to
> reject recipients at SMTP time wherever possible.

if the callout succeeds, why should the secondary accept the message?
that server has no business talking to you if the primary is up. I say
defer it, deny it if the callout returns a negative answer, and accept
it only if the callout times out.

the one failure mode this doesn't address is partial connectivity: the
server can talk to you, but not the primary, while you can talk to
everyone. these days it's so common for spammers to go after the
secondary, I'm convinced the upside of defering valid addresses is
larger than the downside of not handling this split Internet situation.

(qmail users can go stuff themselves... qmail will not reapply the MX
lookup after failure, so it will lock onto the secondary MX if the
primary fails. this issue only arises if the primary and secondary have
temporary problems at the same time.)
--
Kjetil T.