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).

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