Buttondown is designed for minimalist newsletters. You get to avoid complex WYSIWYGs and cumbersome blocks and builders. It's a tool that supports Markdown and enhances it even further to allow you to embed tweets, highlight code samples, and more.
And, of course, HTML is valid Markdown! So if you want to really customize your layout, you're more than welcome to.
Markdown, but super-charged
Buttondown has support for your favorite Markdown extensions (tables, footnotes) and a couple ones I built out just for newsletters.
Preview your writing easily
Buttondown's drafting mode makes it easy for you to view your compiled Markdown as you write, and in your inbox.
Embed gifs and more
Getting dynamic and external content into emails is a pain. Buttondown's rendering extensions make it easy.
Automatic link checking
Buttondown goes through and makes sure you don't accidentally send out any broken links or typos, too.
Wow. Buttondown kind of had me with the Markdown support, but a response that speedy kind of seals the deal.
I like that I can just copy-paste my work from my Markdown text processor and it's ready to go. A lot more tailored to this kind of need than a more general service like Mailchimp for example.