I tried them in 2019, they did (and do, I guess) check all my boxes but it seemed they were moving fast and breaking stuff and IMAP/SMTP in Apple Mail was one of the broken things. Docs were very much out of date, IMAP kept erroring out every few attempts and SMTP never worked at all (their support people couldn't help either IIRC). IMAP/SMTP is a basic requirement for a not-E2E-email provider so that didn't instill much trust. Are they doing better nowadays?

FWIW, I'm a happy customer of https://mailbox.org for more than a year now. I know several other users who are also happy with them.

It's a no-frills, secure setup for a decent price. I definitely recommend mailbox.org.

I've been using IMAP with mbsync and gnus for probably over a year now and I've not had a single issue with it. I'm not the most prolific emailer, mind you.

I've created a trial account and they really have tidied up that part of the experience, they offer a downloadable profile for macOS that will set up Apple Mail, that's really neat and something I haven't seen with other providers yet. Will give them an in-depth check, this looks promising.