Marketo has some time of email reporting tool that I *think* is an integration with Litmus. Litmis does report on the email clients used by your subscribers. If you also happen to be a Litmus customer, they have an email analytics script that you can add as a token to your email in Marketo:
http://developers.marketo.com/blog/integrating-litmus-with-marketo/
https://litmus.com/help/analytics/how-to-guide/
I only know about the integration because they briefly showed it in their 2015 roadmap demo at this year's Marketo summit. I don't think it's included with the basic Marketo license however. Probably an additional add-on/upgrade.