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I've used (am using) Zoho for a domain of mine. It doesn't see a lot of use - but it's worked whenever I've needed it.

Maybe ProtonMail?

It's a decent alternative, but the lack of FIDO 2FA is somewhat disappointing.

I use Runbox. Interface is not as slick but service is excellent.

I mainly use the webclient, the interface for runbox is sadly atrocious (composing a mail even gave me a horizontal scrollbar)