Interesting discussion, this. For a given well-designed MX platform where the MX servers are physically separated and are on different networks with different upstream providers, there are myriad reasons why any two MX servers for a given domain might not be able to reach each other. That means they might see each other as "down" when in fact they're reachable from everywhere but themselves.
There are also myriad reasons why a given client - that is, an "external" MTA trying to deliver a message - might only be able to see one of those two. In the case where the two MX servers can reach each other, but a client cannot, delivery delays could occur. In the more condensed scenario, where there are two MX servers on the same network which share the same upstream there are still myriad reasons why a given external client might only be able to reach one and not another. In the end, however, none of this matters - as long as the MX returns a 4xx response (that is, temporary) then the sending MTA should requeue and retry - and will probably deliver the message later. To go back to the OP's question: you need to check your logs to work out why Exim isn't speaking to the primary MX. Graeme -- ## List details at http://lists.exim.org/mailman/listinfo/exim-users ## Exim details at http://www.exim.org/ ## Please use the Wiki with this list - http://wiki.exim.org/
