How to deal with unroutable addresses on the secondary email server (mx) with exim?

How to deal with unroutable addresses on the secondary email server (mx)
with exim?

I have configured two instances of exim4 on two different Ubuntu servers.
They both accept e-mail for the same three domains. The primary one acts
as the final destination whereas the secondary only relays e-mails to the
primary one.
There is a problem however with unroutable addresses. Every once in a
while the secondary server will receive an e-mail for a non-existent
address but within the proper domain. It accepts this message and then
tries to relay it to the primary server. The primary server however
rejects it with "550 Unrouteable address" thus getting the secondary stuck
with all those messages frozen in the queue.
What is the proper solution to this problem? Should the secondary server
drop messages rejected by the primary server? If so, how do I configure it
to do that?
And how do I do it without losing the redundancy. Let's say the primary
server comes up with a broken config and rejects all messages. I wouldn't
want to lose them. I would actually appreciate if the secondary server
held on to them until primary is restored to its proper config.
Surely, the problem is not new and canonical solutions exist...?