Mailbox.org is based in Germany, which is why you generally cannot trust its feature that automatically encrypts all incoming emails. Other german mail providers, such as Tuta, have already been forced at the direction of government authorities to store every incoming email from certain accounts separately in unencrypted form. Even though Proton also cooperates with authorities, Swiss data protection laws are significantly stricter in this regard. So far, there is no regulation there that requires them to implement a backdoor. They only disclose metadata, the IP address, and the backup email address. Whereas Mailbox would have to forward the entire emails to the authorities.

I see, I don't use email for anything private because even if my mail server stores it encrypted, it passes through potentially multiple other servers on the way which have unknown privacy.

It's just for notifications and newsletters mostly.