Tuta comes with a caveat - you cannot use it in any other mail client (I think there are similar limitations with Proton as well).

Anyone looking for alternatives - stay away from mailboxo.org. It's a pathetic service. Stuck in past (they have a suite that makes you kick a table leg), very disgustingly bad customer service (it's almost non existent), and yeah they use 2FA inside the password.

Tuta is many times better if you can live with not being able to use another client. (They have pretty decent apps on all platforms though)

Do you know anything about another popular German mail provider, mailbox.org?

mailbox.org were good, but they decided to become more than just a mail provider and forced users into other, more expensive plans, adding office and cloud storage.

There is also posteo.de. It doesn't support custom domains, but I use it in combination with simplelogin.io (I think French, but now owned by Proton).

Yeah, I appreciate posteo's stance but when I tried using my ___domain via forwarding service it was a pain.

SimpleLogin, by the way, is now owned by Proton which is run by a founder (CEO?) who is a vocal Trump supporter. Nothing wrong with that of course, just saying.

I have commented just above it. tl;dr - I'd avoid it like plague at this point.