[exim] Try next router, on defer

I have a manualroute router that connects to a server that quite often
sends 45x responses at the end of the data phase, for periods of a few
hours. What I would like if this happens, is for the next router to be
tried instead (dnslookup).

Currently I have a condition in the manualroute router:

condition = ${if <{$message_age}{600}{true}{false}}

This means that if it fails the first time round because of a 45x error,
when the queue runner retries 15 minutes later, it'll skip this router.
That's not ideal though, for several obvious reasons. I couldn't find a
generic router option or manualroute option in the docs that'll let me
do this, is there one?

I understand this is an odd situation, please don't ask me to explain
why the server I'm talking to sends the 45x's ;) It's not time based and
can't be predicted in advance.

Mike