[Exim] Greylisting + multiple MX hosts -> multiple attempts

Earlier today, I spotted a target host using greylisting on us
(hello, Aber, this means you ;-)

I was a bit surprised by the log entries showing that we were making a
triplet of attempts at each retry time.

The answer was that their MX entry pointed to three IP addresses, and
as the retry time came around, we were making three retries in rapid
succession to them. As Chris E said to me, this is in fact correct
behaviour in response to a 4xx status; it just so happens that - in
the case of greylisting - the repeat attempts are a rather pointless
activity.

Probably there's nothing to be done about it, but I thought it might
be worth mentioning here as a possible unintended consequence of using
greylisting. I'm not sure what that means for the kind of site which
have a whole string of MX hosts defined.

cheers