Here's a new one on me.  How does a list owner launch such probes?
This mechanism could be quite useful.
 
----------------------------Original message----------------------------
 
Wed, 5 Jun 1996 15:29:37
 
This message is a "probe" for your subscription to the LSTOWN-L list. You
do not need to  take any action to remain subscribed to  the list, and in
particular you should  not reply to this message. Simply  discard it now,
or  read on  if  you would  like  to  know more  about  how this  probing
mechanism works.
 
A  "probe"  is a  message  like  the one  you  are  reading, sent  to  an
individual subscriber  and tagged  with a  special signature  to uniquely
identify  this  particular  subscriber  (you can  probably  not  see  the
signature because it is in the  mail headers). If the subscriber's e-mail
address is no longer valid, the  message will be returned to LISTSERV and
the faulty  address will be  removed from  the list. If  the subscriber's
address is still valid, the message will not bounce and the user will not
be deleted.
 
The main advantage  of this technique is that it  can be fully automated;
the list owner does not need to read a single delivery error. For a large
or active  list, the manpower  savings can  be tremendous. In  fact, some
lists are  so large that it  is virtually impossible to  process delivery
errors manually. Another advantage is that the special, unique signatures
make it possible to accurately process delivery errors that are otherwise
unintelligible, even to an experienced technical person.
 
The  drawback,  however,  is  that  this  method  lacks  flexibility  and
forgiveness. Since the Internet does not provide a reliable mechanism for
probing an  e-mail address without  actually delivering a message  to the
human  recipient, the  subscribers  need to  be  inconvenienced with  yet
another "junk message". And, unlike  a human list owner, LISTSERV follows
a number of  simple rules in determining when and  whether to terminate a
subscription; it doesn't remember that Maite wrote last week to warn that
there might be  problems with her ISP over the  weekend. In particular, a
common  problem with  automatic probes  is  mail gateways  that return  a
delivery error, but do deliver the message anyway. LISTSERV has no way to
know that  the message was  in fact delivered, and  in most cases  r. the
subscriber is not aware of the  existence of these "false" error reports.
If this  happens to you,  LISTSERV will send  you another message  with a
copy of the delivery error returned by  your mail system, so that you can
show it to your technical people.