There's so much wrong with it, you're better off just getting a server, make it actually encrypted and then, when you do send e-mails, encrypt them, because what they sell you is safety, but that only happens if you send e-mail to other proton addresses, everything else you need to use a encryption key (just like @aoeu explained) and make sure the other side is willing to do it, for registration on websites, e-mails from companies and so on, they'll have them in 'plain text' on their end, so if they do get hacked for example, and get access to any e-mail accounts, they'll see that you're a user there and can track you back on the website itself (generally speaking, of course).