[Exim] mirror MX

I am thinking of implementing a mirror MX setup where two servers,
spatially separated, will both handle and have a copy of the email
for their domains (all other services will be also handled this way,
but this is not an exim problem and it is anyway easier to implement).

The setup would be something like:

DNS:
server A primary MX for domain1, ... domainN
server B secondary for the same domain

but domain1,... domainN are all local_domains in both servers

I would need an extra router, for instance on server A:

smart_route:
driver = manualroute
domains = +local_domains
transport = remote_smtp
route_list = * B
unseen

what would happen if B is down or unreachable when the copy delivery
is attempted? I guess the message will stay in the queue, correct?

And how will I ignore this router for messages that are being mirroed
from the other server? I should probably add a header, that makes me
think all this process should be actually done is a system filter..

Also I would have a special accept rule for local_domains when coming
from the other mirror server.

Is this a sensible approach (once fixed...)?

Thanks

Giuliano
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