On 9 Jan 2004 at 9:18, Harald Tveit Alvestrand
<harald(_at_)alvestrand(_dot_)no>
spoke, thus:
Why doesn't your friend use ETRN to trigger delivery of his queued
mail from his mate whenever he gets online?
He doesn't want his mate getting his mail while he's not available if
he will be available shortly after.
So configure the secondary MX return to 4xx in response to RCPT whenever
the primary MX should be available "shortly after".
Also, etrn/similar don't solve the RBL problem.
For that matter, RBL doesn't solve any problem either. But if you
really want to do RBL filtering on mail relayed to the secondary MX, you
should be able to filter on addresses in Received fields. Sure, it's
dodgy, but if you're trusting RBL you're already demonstrating you don't
care about reliable mail delivery.