[Exim] Wishlist item..

While considering enabling "dns_again_means_nonexist", it occured to me
that while one would want to retry for a little while on legitimate
temporary DNS failures, a lot of spam-domains have permanent DNS
failures.. It would be nice to have a "dns_again_means_nonexist_after",
which would work similar to the "ignore_errmsg_errors_after"
setting.

By the way, bravo on SMTP callbacks.. I've enabled them on my private
(low volume) server about a week ago, and I've already eliminated
reception of quite a bit of crap..