I think mailbox.org is the better choice because they actually have a DMARC policy. With posteo, anyone can spoof posteo domain and the remote mail server has no way of checking that.
It’s a nice option to have, which is not possible with Posteo. They claim the reason is “for privacy”:
Can I use Posteo with my own domains?
No. We are an email provider with a particular, privacy-oriented model – and this is not compatible with incorporating own domains. One of our emphases is data economy: we do not collect any user information (names, addresses, etc) of our customers. We always answer requests from authorities for user information in the negative. On the other hand, own domains need to be registered to the name and address of a person. If you were able to use own domains with us, this would affect the entire concept of Posteo: we would need to start saving user information for all customers who use their own domains with us – and to provide these to the Federal Network Agency to be provided on request to the authorities.
Even if only the MX record pointed to us, we would still need to store the assignment of the domain in your Posteo account as user information. Thus we would possess your user information and be required to give it out. For this reason, we have decided not to offer this possibility and instead to use data economy.
They claim this is because of their “anonymity” however they have no anonymous payment options like Monero. It’s purely a “trust” thing that you “trust” they can’t reverse the transaction anonymization. They also leave out that there are anonymous “proxy owners” like Njalla for example who could register a domain for you.
The calendar/contacts encryption with Posteo, really isn’t anything special, and is a bit like Startmail’s vault, in that it’s decrypted server side when you login. We don’t call this “zero access” encryption.
Protonmail and Tutanota offer encrypted calendar, contacts, so they do that have over Mailbox / Posteo, but of course you have to use their client (or the bridge with Proton Mail).