Sendfox is a tool by the folks at AppSumo that's focused mostly for existing customers. If you're looking for a barebones tool that allows you to segment your list and create some basic automations while staying within the AppSumo, Sendfox may be the tool for you.
However, it lacks a lot of the major features you may be looking for in a newsletter, like support for archives, scheduling, paid subscriptions, or advanced analytics.
Buttondown doesn't block its polished editor behind a $49 fee, because I think the most important part about building a newsletter is the writing process.
If you're looking for a newsletter tool that can handle your workload while being easy to integrate with, Buttondown is the tool for you!