Here at Ironpaper, we work with many different mass email campaign platforms. One of our favorites is MailChimp, and even more so now after their latest update. They've added an amazingly intuitive, and fun to use, template editor that allows users to drag-and-drop content areas and populate them as needed. Once a content area has been added, it is fully customizable - including the stylesheets - without having to know any code (of course you can, if you'd like). And the template preview updates instantly as you make changes.
Another amazing feature is that it automatically makes your templates responsive - ie they'll display seamlessly on mobile devices or tablets. When you preview your design, it shows the normal view side-by-side with how it will look on a smartphone - and you can rotate the mobile preview to view it both horizontally and vertically.
Finally, another thing we love - they've integrated the Aviary image editor to allow you to actually edit images and designs after uploading them within the browser. You can crop, resize, adjust the color, and even add text to images after they've been uploaded.
The project was codenamed "Neapolitan"; the analogy being between the three "blocks" of flavor in the ice cream and the different content blocks that can be easily moved around.
Check out this step-by-step example (https://blog.mailchimp.com/introducing-mailchimps-new-editor/), or watch their video tutorial (https://kb.mailchimp.com/article/how-do-i-create-a-campaign-with-the-drag-drop-editor) to get an idea of what's possible. Or just log in, create a new campaign, and start trying it yourself - it's so easy, you'll be creating amazing templates in no time without having to write a single html tag.