[Exim] Falling Back

We have a VPN (not directly connected to the internet) with three
locations. Each location has an ADSL connection to the internet. On each
location exim is sitting and waits for mail which it then forwards to the
central mailserver (using the domainlist router). Works like a charm.
I am playing with a fallback strategy to get our mail out should one of the
three ADSL connections be unavailable for some time. Basically what I did
was adding a fallback_hosts entry to the remote_smtp transport.
Unfortunately it also falls back if the remote smtp is unavailable or the
connection times out. It should only use the fallback_host if the link goes
down. How could I do that? Else I risk a mail loop between my three exims.

Then, I noticed that fallback_hosts needs a DNS entry to resolve - just
putting the name into the hosts file does not work. Is this by design?
Would adding a 'byname' tag help?

-- Ben

PS: exim rocks. really.