Rodrigo Severo wrote:
> [...]
> I'm wondering if it might be better to:
> 
> a) not to deal with it at all because there might be so few cases

Appealing as this option can be, to make an informed decision we still
need a sharp view of what we don't want to deal with. Your clear analysis
of the abnamro.com DNS setup proves that Courier has a problem with broken
hosts with badly configured DNS records. Thanks for that.

> b) try to detect the specific situation when this problem occurs with
> simple non-obstrusive code and use a more convoluted strategy to choose
> MXs only then;

That apparently implies saving host status information. Accompanying
questions are where and how to store it, when to delete it, etcetera.
I would opt for a global db rather than each single control file.

That is not extraneous to an MTA business. Sendmail, for one, does so:
"  [HostStatus] allows mail to be queued immediately or skipped during a queue
"  run if there has been a recent failure in connecting to a remote machine
"  <http://www.sendmail.org/~ca/email/doc8.12/op-sh-2.html>

> instead of changing the way Courier chooses MXs for every situation as
> it's current implementation is quite appropriate most of the times.

Storing host status only when the MTA detects a problem makes sense.
Additionally, it can be further restricted to the case when the target
MX is not the only one with its weight. To reduce the amount of new
problems introduced by the new strategy, the host status can be deleted
when the set of MXes changes, when the target MX changes address, when
a timeout expires, and similar euristic criteria. In such environment,
connection timeout failures can be distributed evenly among sibling MXes.

Is that the kind of solution we're talking about?


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