If you want a Superhuman-like interface, that is running in TUI with neovim as a composer and a modern neomutt as a reader, check out neomd [1]. I'm the creator, so I'm biased, but I replaced my previously used HEY email with it. It has a screener and a GTD workflow built in.

Just in case, it works with any SMTP/IMAP setup like Fastmail, or any other. Proton mail works as well but need a little more to setup initially, even gmail (but much slower as the article explains, I noticed that too)

[1] https://neomd.ssp.sh