en 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.

It generally means that someone has included something in the body of
the mail that LISTSERV thinks look like legitimate mail headers.  This
happens in some mailers when people forward a message to a list without
including a > or something in the first space, or if they are illustrating
a point and use a "From:" (or other legit mail keyword) in the body of the
mail they're sending.