[Exim] dns_again_means_nonexist = how to use this

The docs are a little confusing in that it indicates the will bounce
email. What I'm looking for is a way that if I do a dnsdb lookup and the
dns fails it returns nothing - and then I process that - not bounce it.

The problem with doing dns lookups is that I don't want messages stuck
in a queue forever retrying. I want to just get the info and move on
with a failure returning that the lookup failed. I'm just setting
headers with this info for spam assassin to score.