I’m a posteo customer since many years, so I can tell you why I’ve chosen it, but can’t compare it to mailbox.org too deeply

• posteo has a way how they cannot link your payment to your email account. I can’t find something similar at mailbox.org.
• It sounds like posteo takes sustainability serious, they even mention office Furnitures. Mailbox.org’s information is not that detailed.
• posteo.de does not save users IP addresses. Except they are forced by law.