[Exim] Using primary and secondary smarthosts.

Hi,

I currently have a site where there are multiple internet links through
different providers (DSL, ISDN and modem).

I have tried unsuccessfully to create a router that will use a 'primary'
smarthost (through the DSL link), but then change to relaying mail through a
'secondary' smarthost (through a ISDN or modem link) - and then change back
to the 'primary' smarthost when the DSL link is back up (exim is is own
server, the DSL link has its own routers, and the ISDN and modem links share
another router).

The router I have been playing with is:

mailroute:
  driver = domainlist
  transport = remote_smtp
  route_list = "* primary.smarthost.com bydns_a; \
                * secondary.smarthost.com bydns_a; \
                * tertiary.smarthost.com bydns_a"
end

Essentially is appears that when the primary smarthost is unreachable, only
some mail will be sent through the secondary smarthost (whereas other times,
mail just sits in the queue). When this happens, exim also does nto appear
to change back to the primary smarthost unless the queue is empty??

Can someone please point down the right track (and please least explain how
exim chooses between routes to the same destination)?

Thanks in advance,

Darren Mackay