Mailmate (https://freron.com/)

Have always been surprised it doesn't get more attention on this forum. It's a paid app, and it shows in the quality. Very smart and dedicated single developer (this is feasible for an email client since the protocol is stable and doesn't change very much), an active mailing list and user community. Clean with few bugs in my use. Definitely aimed at developers and people who want to peer into the internals of email.