Q0613
Question
I have set fallback_hosts on my smtp transport, but after the error sem@example.com cannot be resolved at this time Exim isn't using them.
Answer
fallback_hosts works only if an attempt at delivery to the original host(s) fails. In this case, Exim couldn't even resolve the domain example.com to discover what the original hosts were, so it never got as far as the transport. However, see FAQ/Routing_to_remote_hosts/Q0315 for a possible solution.