[exim] retry with a different router if delivery failed

Hi,

I have a backend system which might not be available sometimes. Usually I can
forward my messages from Exim directly with SMTP but sometimes this will fail.
Currently Exim just puts the message in a queue and applies the regular timeouts.

Ideally I'd prefer if Exim could place a message to a local maildir in case of
a delivery failure (so the backend system would fetch these later using IMAP/POP).

Is that possible in Exim? I checked the "retry" configuration as well as
Exim's "events" but so far I didn't see how to use the built-in mechanisms.

Felix