I don't do often E-Mail design, but when I do, I use the grunt-email-workflow
It is super neat. You can style the e-mails with sass and the templates are made with handlebars, so you can work easily with partials and components.
The grunt task compiles your sass to css, handlebars to html and then inlines the styles automatically.
You can also add some cnd to it will upload images etc to it and you can add a mailgun api key so you get tests emails. Or test them with Litmus,