Zero-access encryption means that the protected contact fields are encrypted in such a way that only you are able to decrypt and read them, not even ProtonMail can read them. While you can't encrypt the e-mail address of a contact (since ProtonMail needs to know where to send the email) you can encrypt other fields such as Address, Phone Number and Notes.
The goal of this feature is to provide another level of security for journalists and others who need to keep the identity of acquaintances (e.g. sources) as secret as possible while still being able to contact them.
In additional, digital signatures are utilizes to verify the integrity of contacts data, including the e-mail address. Verified fields are denoted with a tick. If verification fails (e.g. due to tampering), a very prominent red-background warning will display.
More information: ProtonMail
Written by: James Delahunty @ 22 Nov 2017 7:58