Email marketing for many businesses is a significant driver of traffic and revenue and there are some excellent email marketing campaign tools out there such as MailChimp and Campaign Monitor which offer the ability to create and send effective emails. The problem with these tools is the reporting options that they offer, although impressive, is limited to email interaction data such as open rates, clickthrough rates, bounces and so on. Conversion, sales and revenue data isnt readily available however except in limited instances e.g. I believe MailChimp has a module that will work with Drupal sites to measure sales. Our good old friend Google Analytics enables you to track email sales and traffic.