I can't praise MailMate[1] enough here: A lean, no-fuss mac email client with first-party support for Markdown (with syntax highlighting!), and plenty of other goodies.

Cherry on top, its developer is very active, and keeps a regularly-updated blog[2], talking about email server quirks and many other interesting technical bits.

Note that I'm not in any way affiliated with the software – just a very happy customer.

[1]: https://freron.com

[2]: https://blog.freron.com