[exim] Routing failed deliveries through an ESP

In an ideal world, I'd have a single dnslookup router that happily
delivered mail all day long. But host reputation is a fickle beast, and
it's painful to have mail sit around deferred or frozen until I get our IP
taken off the DNSBL list of the week.

As a solution to this game of whack-a-mole, I'm curious if I can configure
Exim in such a way that it attempts a first delivery through the normal
dnslookup router, and if the mail is rejected, retry with a router that
sends it through an ESP (Mailgun in my case) which has way more IPs and
resources to keep them clean.

How might I configure my routers to ignore an initial 5xx response from the
first router and attempt another (and maybe future) deliveries through an
alternate router?

Thanks!
Lance