It is E2E between proton users.

If you want to encrypt it to other users, you can. Although you have to set a password for your email.

Then the receiver will only get a proton website link, which if they click, they have to put in your password and then they can view it.

I think that is the only way to encrypt to any arbitrary mailbox. 

 I do have a password to my Proton email but didn’t realize I needed to give that password to whomever (with a different email account) received my transmission. Thanks so much for weighing in on this important topic. 

 No, no! 

 When you create an email, and you want to send to a non-proton user, if you click the lock icon, you have to set a specific password for that email. Then it will be password protected/encrypted, and only openable with that password. 

NOT your account password. You shall not share that with anyone! 

 Yay!  Thanks for clarifying. I was not too keen on sending my main password to my ProtonMail account out. 

 No problem. I am glad we discussed this before you shared it. 💜

Rule 1: Never share your password with anyone!

If you have to. They try to scam you, or you need to use a different product 🤙