I decided to go for mailbox because 1) I know that Germany at the moment has still one of the best regulations about data protection (although I fear this is going to change in the next few years), 2) it provides some features others don't (like protonmail, and startmail). It was worth a try at least, so I decided to use the 30 days trial account.
It looked really good and promising: nice UI, clear documentation, cool domain name if you don't want to use your own. It provides also Office-like, calendar, and storage features. Therefore, I made up my mind, and I was determined to become one of their paying customers. So, I put 12 EUR(1 EUR/month) on the account. A few hours later I found out[0] that mailbox.org is offered by a politically motivated provider called JPBerlin [1]. I sent the cancellation request, and so far my account is still on hold - I could revoke the cancellation, though. An email received after the cancellation request says "please allow us a couple of days". Sure. It's just they took 1 EUR from the account, although I had the cancellation request sent like 10 days before the end of the trial period.
In the end, I would like to say that as a service it looks promising. However, until they stop with their political involvement, I think, not many people will use it.
[0]: http://www.emaildiscussions.com/showthread.php?t=68527 [1]: https://www.jpberlin.de