[Exim] fallback_transport?

This is sort of an extension on C0303

(How do I configure Exim to send all non-local mail to a central server
if it cannot be immediately delivered by my host? I don't want to have
queued mail waiting on my host.)

of the FAQ, which points the questioner in the direction of
fallback_hosts..

What I want, is to have any mail which cannot immediately be delivered
by the regular SMTP, to go to another transport (which will not be an
smtp transport). Any suggestions?