[Exim] DNS Issue - How do I get around this

I'm using this ACL to get rid of spammers who are trying to impersonalte
themselves as being one of my domains - and it works pretty good .....

deny    message    = Message Rejected - Host impersonating
[$sender_helo_name]
    condition = ${lookup{${lookup dnsdb{a=$sender_helo_name} {$value}}}
lsearch \
                {/etc/exim/badheloip} {true}{false}}

.... except .....

When I run into DNS problems on dnsdb{a=$sender_helo_name} it "fails to
expand" because the lookup fails and it defers which creates a lot of
stuff queued.

I've asked for this feature before but I REALLY NEED IT NOW. A way to
real with DNS looking failures other than DEFER.

Can anyone suggest a way around this?