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

Electronic mailing lists
Parenthetical citations–reference list 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 date of the posting, and the name of the list. Omit e-mail addresses. Such items should usually be cited only in a parenthetical citation, with the elements separated with commas. You generally need not include them in your reference list, although you may choose to include a specific item that is critical to your argument or frequently cited. In many cases, you may be able to include some or all of this information in the text.

P: (John Powell, October 30, 2004, e-mail to Grapevine mailing list)

In an e-mail to the Grapevine mailing list on October 30, 2004, John Powell declares . . .