Our philosophy on the inane 'after 3 days...' non-helpful messages is to
just make a note of the affected address.  If we get more of those over the
next few days we remove the address from all of our lists.  If the removal
notice happens to get through (we DON'T do a quiet del) it generally gets
the attention of the person at the other end.
 
   In the Field of Dreams parlance this would be the "Ease my Pain" approach.
If you do nothing to prune your list you will get more and more pain.  We take
a similar approach to other "soft" errors (disk full/quota exceeded).  For
messages which say "no such userid exists" we take their word for it and
nuke the address right away.  But in each case we make a note in a log
(diary) file so if the user asks we can explain what happened and encourage
them to look up the folks who run their system.
 
       Marty