If I may add to this thread, the key purpose of a citation is so that later
scholars/researchers can verify the source of the information.  From that point
of view, I believe it highly unlikely that in ten years hence, anyone will be
actually able to retrieve a "Museum-L" e-mail message to verify such citations.
 
The best citation therefore, provided that you have a printed copy of the
e-mail and keep it with your research papers, is to say:
 
Personal correspondence with the author, John Smith, April 15, 1994 to Jane
Doe, Museum-L electronic mail list.
 
--Dean DeBolt, University of West Florida