I receive email from a discussion list and I set the discussion list address as exclusive sender.
I noticed that sometimes a message decremented the counter. I recently compared the header of all received messages to see if I could find under which condition the message passed unrecognised.
It seems to me that if the message is send to multiple recipients and that the exclusive sender address does not appear as first in the recipients list, it is not recognised as "exclusive" and therefore decrement the counter.
Can someone confirm this or should I look further?