We use Mixpanel to send out transactional emails based on user actions (the ones that make sense to A/B test - welcome emails, inaction emails etc). A/B testing would be great, but it kind of feels like double work adding user event tracking code for Userfox too - maybe you could integrate analytics providers such as Mixpanel?
i hope one day mixpanel will offer some manner of connector to enable this functionality. we'd love to do this.