A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Electronic mailing lists
Notes-bibliography style: citing specific types of sources
Part II. Source Citation

To cite material from an electronic mailing list, include the name of the author, the name of the list, and the date of the posting. Omit e-mail addresses. If the material is archived, also include the URL and the date you accessed the material. Such items should usually be cited only in a note. You generally need not include them in your bibliography, although you may choose to include a specific item that is critical to your argument or frequently cited.

N: 17. John Powell, e-mail to Grapevine mailing list, October 30, 2004, http://www.electriceditors.net/grapevine/issues/83.txt (accessed October 12, 2005).