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.

• posteo.de for Mail (1€/month)
• addy.io for mail aliases with custom domain (1€/month)
• 2 domains at gandi.net (DNSSEC, U2F, 🇫🇷)
• 2 small VPS + 1TB Storagebox at Hetzner
• 1TB Seedbox @ hostingby.design 🇩🇰
• Encrypted Notes with nice, fast editor @ notesnook.com (Essential Plan - 1,73 €/month)

… and my employer pays my Kagi.com account 😁