Another mailbox.org user here. I did the same switch as @caravaggio@feddit.nl around the same time and I can highly recommend as well. Setting up custom domains is also not hard and well documented in their knowledge-base. I am also using it for calendars and contacts with no issues at all. A plus of their premium (3€/month) plan, apart from custom domains, is that you have access to a series of other things (appointments, videoconferencing etc.) which are a nice thing to have if you need them (as an occasional teacher in academia I enjoyed having the options, especially since I could avoid Google/Microsoft stuff).

The only annoying thing is how they handle 2fa login on their website. I rarely need to login, but when I do I always suffer.

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Kvaesitso user here too, can confirm. It took two tries for me to really appreciate it and set it up as I wanted, but now I don’t look back anymore.