You can actually inline CSS with tools (here’s one by Zurb).
For responsive emails, inline the styles you’re using for the default view of the email and then add everything else in a
<style> tag with a media query.
You miss out on quite a few clients (gmail, outlook, some others) but it’ll work with iOS/mac mail which I think has a decent user base still.
Regardless of how you approach it you’re going to have problems, I highly recommend using a service to test what you’re sending in a bunch of clients. I use Email on Acid at work.