Why not consider it essentially the same as a magazine article?  The
analogy is particularly apt on USElessNET, where everything is an
"article", but I can see that extended to LISTSERV mailing lists,
too.  Now if it's a personal letter sent from me to you, you'd probably
want to cite it that way (with appropriate notation that it was an
e-mail, rather than USnail, letter), but list messages might go like
this:
 
Brindle, Nathan.  "Re: Net message citation" (December 6, 1994, 18:25:27
    EST:  e-mail posting to [log in to unmask]), 2-6.
 
My thought is that the time would be critical--I might respond more than
once to messages with that subject heading.  The last item would be the
specific lines of the message quoted, in the example, lines 2-6. It might
even be appropriate to quote my Internet address.
 
Your mileage may vary.  My Turabian is buried in a box somewhere, waiting
to be moved.  :)  But remember that the point of a citation is to allow
the reader to find your source if he/she feels so moved.  The information
above would make it quite easy to find the cited message in an LDBASE
search.
 
Nathan