Protonmail brags that it is “NSA proof”. It does have some interesting features, and is based in the time-honored nexus of privacy: Switzerland (remember the swiss numbered bank account? no names necessary !).
Self-destructing messages (you schedule the date / time) are a nice feature. Features are listed on their product page; I won’t bother to repeat them. Early on, a highlighted feature was the use of separate passwords for the account and for the contents. The idea is, the people who run protonmail do not have access to the password for your messages, so they cannot leak your messages to anyone: hackers or law enforcement, or insider leakers alike. More recently if you sign up that has collapsed into one password (for convenience), although you can still change the settings to use two separate passwords.
Recently protonmail opened a doorway in the dark web at https://protonirockerxow.onion, which helps make your location less traceable over the TOR (the onion router) network.
A good analysis of the security of protonmail is here.
Notice the email address on the Contact page, is on protonmail, so I use it myself. VCreate your own free account, and send me a note !