Mixpanel, an analytics-focused startup whose backers include Y Combinator, Michael Birch, and Max Levchin , is getting into email analytics today with the launch of a new feature that allow publishers to track a variety of metrics for email correspondence. The startup, which now tracks over 1 billion actions per month, allow developers to track a variety of user actions, including the number of pages a user has viewed, iPhone app analytics , interactions on Facebook apps.