Looks solid! Even the free version!!!!
I asked them and this was their response:
The encryption level that is used between all types of ProtonMail accounts is the same. Messages sent between ProtonMail users are end-to-end encryption.
End-to-End Encryption messages are encrypted at all times
Messages are stored on ProtonMail servers in encrypted format. They are also transmitted in encrypted format between our servers and user devices. Messages between ProtonMail users are also transmitted in encrypted form within our secure server network. Because data is encrypted at all steps, the risk of message interception is largely eliminated.
Zero Access to User Data. Your encrypted data is not accessible to us
ProtonMail's zero access architecture means that your data is encrypted in a way that makes it inaccessible to us. Data is encrypted on the client side using an encryption key that we do not have access to. This means we don't have the technical ability to decrypt your messages, and as a result, we are unable to hand your data over to third parties. With ProtonMail, privacy isn't just a promise, it is mathematically ensured. For this reason, we are also unable to do data recovery. If you forget your password, we cannot recover your data.
Demo showing email cryptography End-to-end encryption means that no one but the intended recipient can read the message
Open Source Cryptography
Time-tested and trusted encryption algorithms
We use only secure implementations of AES, RSA, along with OpenPGP. Furthermore, all of the cryptographic libraries we use are open source. By using open source libraries, we can guarantee that the encryption algorithms we are using do not have clandestinely built in back doors. ProtonMail's open source software has been thoroughly vetted by security experts from around the world to ensure the highest levels of protection.
By using open source encryption libraries, we can help guard against back doors designed to compromise your privacy.
All user data is protected by the Swiss Federal Data Protection Act (DPA) and the Swiss Federal Data Protection Ordinance (DPO) which offers some of the strongest privacy protection in the world for both individuals and corporations. As ProtonMail is outside of US and EU jurisdiction, only a court order from the Cantonal Court of Geneva or the Swiss Federal Supreme Court can compel us to release the extremely limited user information we have