[deleted] It's important to note that Proton is only "zero-access" or "zero-knowledge" so long as they want to be. It's technically possible for them to change their client code (the code loaded into your browser or app and used to decrypt your private key using your password) at any time to make it exfiltrate your key, and then use your stolen key to decrypt any messages in your mailbox which have been encrypted with the corresponding public key. In a typical setup where the user has only generated and used one key pair, this probably means all of them.
An attack like this was recently used on a Tutanota user, where law enforcement compelled Tutanota to modify the client code served to a particular user suspected of being involved in criminal activity, and they were able to retrieve the key and subsequently access the user's messages. Proton claims this type of attack is not permitted under Swiss law, but if you're an at-risk person trying to use strong encryption, you probably shouldn't rely on one company's interpretation of the law to keep you safe.
Proton is not a magical vault for your email like their marketing might suggest; they use PGP under the hood and their implementation suffers from all of the same security drawbacks as using PGP in an ordinary mail client: Subject lines, attachment names, sender, receiver and date metadata are not encrypted, there's no forward secrecy (stolen key = access to all messages), and worse, you can't store and manage the keys offline on a trusted machine. You have to trust the JS code loaded from a Proton server each time you want to access your mailbox.
Posteo does server-side encryption of individual mailboxes, which means the key can be more easily stolen while you're logged in, but data is still protected from passive attack while you're logged out.
Overall, both companies likely do more harm than good: They promote the idea that email can be made secure and private by bolting bad cryptography and cruft onto an ancient system, and this hinders our ability to move away from said system.