Be careful with mailbox.org and their "your contract period ends soon" email. It actually means "pay us or your data will be irrevocably deleted under 60 days". The mail sounds inconspicuous enough, is rather verbose, and even contains the phrasing "you may silently ignore this email". And you will not be getting a single warning before your data is entirely, irremediably deleted.

And even if you only wait 30 days, not 60, your account gets deleted (but not your emails), so you lose any and all ways of contacting their support (rescuing your emails after that gets much trickier). Speaking of which, make sure you use a widespread browser on a computer to use their support platform: otherwise you will get a visual confirmation that a ticket was created, but none will ever be.

TL;DR: mailbox.org good, but (A) make absolutely sure you always have up to date local backups, and (B) beware of the unexpected caveats and the clumsy, confusing wording.