Geert, I didn't have <your> problem, but Friday morning I awoke to find
out that a Microsoft Exchange server in Brasil was bouncing acknowledgements
back to WINNT-L from a subscription that dropped mail into an Exchange
"public folder".  This started about 2AM EST and by the time I got to
it 7 hours later, WINNT-L had completed 72 iterations of this loop and
held itself (Daily-Threshold= 75).  I was able to get into the spool
and delete 247 more mails from the account before freeing the list
(after serving out the offending address and deleting the subscription
from the list).
 
There is at least one other "public folder" subscribed to the list that
doesn't do this, so I'm guessing that the idiot who set up this particular
subscription didn't bother to turn off acknowledgement mails for that
folder.  Loop checking apparently didn't catch it because each piece
of mail was slightly different (time of day, message ID, etc.).  Thankfully
I didn't succumb last week to the temptation to raise the daily threshold
to 100 on that list...:)  Anyway, beware the Exchange Public Folder
subscriber.  He can bite pretty hard.  Good excuse to set Default-Options=
Review on open subscription lists...
 
Nathan