Mailchimp has a feature that gives you stats of the top email clients your contacts are reading your emails in. This is very helpful for formatting and prioritizing optimization for different email clients; for example, Outlook is a nightmare for CSS spacing and if I knew that a large majority of our contacts were using it (which I suspect), I would focus more energy on that design process, whereas if a negligable amount of contacts were using Outlook, I would make it less of a priority. Thanks!
