[exim] A kinder, gentler dns_again_means_nonexistent?

I have an exim4 (4.68) server that gets most of its mail forwarded
from another server ("edge MTA"). I notice there is a certain number
of (probably spam) senders who always defer, I think because their DNS
servers return SERVFAIL to MX lookups. I would like to reduce the
number of needless defers back to the edge MTA.

Can anyone think of an easy way to to start rejecting a sender after
it defers X times in Y hours? I guess eventually the edge MTA will
give up, but I can afford to be a lot more agressive than they can in
terms of what I reject.

I could imagine torturing some greylist daemon into doing what I want,
but I'm not sure its worth the trouble.

I have read about dns_again_means_nonexistent. This seems to say that
if the DNS server is offine, the message is rejected. Does anyone
have any experience using this option in a small, personal server
setting? Any sense of how many false-positives this gives? To give
you some idea of my risk tolerance, I am OK with rejecting mail from
servers that reject callouts from <>.

David