[Exim] Problems with relaying with DNS errors

Hello,

I'm having a problem: I'm using sender_*_callback functions and providing
relaying for a domain whose ISP has been RBL'd. Now, this generally works
fine, but this morning we had some DNS errors, and the callback features
stopped working. As a result, the mail that was supposed to be relayed
was being rejected instead. The significant lines are:

sender_verify_hosts_callback = !a.com : !b.com : ... : !relay.com
sender_verify_callback_domains = !a.com : !b.com : ... : !relay.com

so the relaying host shouldn't even be subject to these checks, but they
are, and they fail. This is an even bigger problem because many of these
messages are automatically generated (password requests, receipts, etc)
and cannot be easily resent.

Is there a way to freeze these messages instead of rejecting them during
these times of DNS woe?

Thanks in advance,
Christopher