What exactly is a "delivery failure"? My spamfilter silently discards
all sorts of messages, but for obvious reasons it's not going to tell
the "higher level" about it, and by consequence no notification is sent.
Should this be regarded an "exceptional event", even though it is often
more the rule than the exception? Or should "delivery" to /dev/null be
regarded as a successful delivery?
Tough question. I see "exceptional event" as being unavoidable loss of data
- i.e. disk crash or the like. I guess that silent spamfiltering is itself
against the spirit of that, and I'm not about to argue that it shouldn't be
done. However, as I see it there is a difference between individual users
(or indeed administrators) choosing to delete mail silently and the working
group issuing a recommendation that mail should be deleted silently; the
problem of antivirus bounces can be solved without resorting to that.