Here’s how you cite an online posting (according to Gibaldi’s MLA Handbook, p 233):
Author’s Name. “Title of the Document.” Online posting. The date when the material was posted (day month year). The name of the forum. The date it was accessed <website address in brackets>.
For example:
Lavagnino, John. “OCR and Handwriting.” Online posting. 7 May 2002. Humanist Discussion Group. 24 May 2002 <http://list.village.virginia.edu/
lists_archive/Humanist/v16/0001.html>.