[Exim] Temporary defer on callouts

I'm now rejecting based on a failed sender callout. I'm using defer_ok on the
callout which does allow a few bad apples through. Are others that are
rejecting by sender callouts also using deny_ok?

I'd like to deny on timeout/temporary error, but I think that might be
too extreme. So, I'm wondering if it would be possible to somehow give
a temporary error, but cache the event and if they try again in an hour
or longer then accept.

The idea is that spammers may not try again on a temporary error, but
legitimate mail would try again.

Is something like that possible -- or even a good idea?

BTW -- I notice in my logs that many of the rejects are three at a time.
That is, I see this listed three times in a row in the log:

rejected RCPT <ted@???>: Unrouteable address

Is that just the way the spammer/virus is programmed to try? Or is
there another reason for three attempts in a row?

--
Bill Moseley
moseley@???