We had a serious mail loop going here last week between LISTSERV
and a Novell Groupwise system in New York state, OSC.STATE.NY.US.
Some user there had set up a vacation program that apparently
responded to every message with one that said "I am on vacation
until ..."  and maybe a blank message for good measure.  Somehow
it got started on this with LISTSERV itself (rather than a list)
and it continued because "I" is an abbreviation for the LISTSERV
INFO command.  As a result, the user was not served out after 20
invalid commands.  Nothing was sent to the LISTSERV postmasters.
By the time we discovered it, there were several thousand of
these messages (and blank messages too) mixed in with valid files
in LISTSERV's reader.  I served out the address, so that LISTSERV
would not reply, but they continued to come for almost a day,
until we were able to contact the people responsible for the
Groupwise system.
 
Eric provided a change to LISTSERV to remove "I" as an
abbreviation for "INFO".  After we applied that change, LISTSERV
stopped a very similar loop that was about to start with a
completely different Groupwise system, but also involving a
vacation message starting with "I".
 
So the fix was very useful, but it still seems like LISTSERV's
loop detection for command jobs could use some improvement, such
as limiting the number of command jobs accepted from any one
address in a day.  It also seems like it would be a good idea to
count an empty message as an error.