[. . .]
>I looked at my "error" folder (my mailer filters out error messages and
>puts them in a folder) and sure enough, there his message was, with a
>head note saying:
>
>> The enclosed message, found in the CASLL mailbox and shown under the
>>spool ID
>> 11259736 in the system log, has  been identified as a possible delivery
>>error
>> notice  for the  following reason:  "Sender:", "From:"  or "Reply-To:"
>>field
>> pointing to the list has been found in mail body.
>
>All I can figure out is that he inadvertently left the message he was
>replying to at the bottom of his own message.  It was marked out with >
>at the margin, but I guess LISTERV is ignoring that?  Is there any way
>to avoid this?  (I'm not going to be able to train my subscribers to
>edit out the To: lines in the included messages they don't even know are
>there . . . )

You don't have much choice.  LISTSERV intercepts these messages because
having lines pointing to the mailing list in the body of a message to the
mailing list is often a sign of a mail loop.  It's possible to disable mail
loop detection to varying degrees, but it's usually a really bad idea, and
I wouldn't take that step without consulting your LISTSERV maintainer.

My usual tactic is to redirect such messages back to the people who
originated the intercepted message.  That way they either figure out the
solution themselves or they go to the trouble of asking me about it
personally.

It's not unreasonable to ask people to be somewhat familiar with their
email client, and in the long term it probably saves them some trouble.

-jwgh