Joni Halabi looks at a bunch of options: Revue, MailChimp, Emma, MailerLite, and ButtonDown. ButtonDown was the best choice for her. Pretty interesting how they offer a physical mail scan-and-forward service to all users.
At CodePen, we hand-roll a whole fancy CMS for newsletters and ultimately send them via SparkPost. In my CSS-Tricks days, I wrote newsletters in WordPress and sent them via the MailChimp RSS integration. If I was sending a newsletter on behalf of a small business with a WordPress site again, I’d consider MailPoet for sure as I like the idea of using the WordPress editor for all content needs. Although I do wish it was block-based rather than their own editor.