Just a guess, but are you using some kind of filter to catch listserv
commands before they go over the list.  I'd like to know more about how
that works.  It looks like it just scans the first line and if it
finds a listserv command, it bounces the message.  Try starting the
message with something like.  "If you are ready to leave HISTARCH, you
should send the following command to listserv ..." so that the words
signoff and unsubscribe do not appear on the first line.