This can happen if the message passes through an Interscan Viruswall
before leaving the user's domain.  The Viruswall wraps the entire message
in a MIME wrapper that LISTSERV can't see into, so you get the "no command
was found" error.  That may not be the only case where this can happen
but it's one I've seen recently and have been able to duplicate.  Typically
the problem is found with MIME multi-parts that have been wrapped in the
Interscan wrapper.  LISTSERV's MIME parser can't parse the original MIME
parts out of the wrapper and that is where the problem arises.

(FWIW this particular problem has been solved for 1.8e.)

On the other hand--why was the user's vacation program sending the mail
back to LISTSERV and not to one of the owner- mailboxes (which would be
referenced in the RFC821 MAIL FROM: header) like it is supposed to do? :)

Nathan