We try to categorize the bounces by content (that is really fun!).
Anyway, any bounce which says "user unknown", "address invalid" or
something like that is taken at face value.  We are hoping when we redirect
the user who is burned by this to the system administrator it might make
mail software just a bit more robust.  Taking a default of "user unknown"
because a disk is down is not a good solution!  ;-)
 
   For things like disk quota exceeded or the infamous "X.400
congestion" messages we do try to let those go for a few days.  (We
have a log we can update with automatic date/time stamp and about 60
characters of text to remind us when we started seeing the disk quota
errors).
 
   The real problems come with cryptic messages (as a class my favorites
are that the error is unknown but it is number 327 or whatever... ;-)
 
     Marty