I bet y'all know that apps like Campaign Monitor and MailChimp have visual email builders built right into them. You drag and drop different types of content right into a layout. You edit text right within the email. It's nice. It's a lot nicer than editing the quagmire of HTML underneath, anyway!
But not everybody needs all the rest of the features that those apps bring, like list management and the actual sending of the email. Perhaps you have an app that already handles that kind of thing. You just need to design some emails, get the final HTML, and use it in your own app.
When I was looking around at email tooling, I was surprised there were a good number of apps that help just with the visual email building. Very cool.
I haven't used any of them extensively enough to make a firm recommendation, but I've been dabbling and I like that they exist and that there are options.