RE: Who's Been Reading Your E-Mail?; A Simple Solution for Better E-mail Privacy and Security
Keep in mind that emails you send FROM protonmail to NOT PROTONMAIL may or may not be encrypted in transit (depending both on how tolerant protonmails' servers are to non-TLS-speaking smtp endpoints and on the configuration of the remote smtp server in the target domain).
They will definitely NOT be encrypted in the other persons' email provider.
If you want to achieve that, then you need PGP.
Windows: https://www.gpg4win.org/
Mac: https://gpgtools.org/
Linux: https://gnupg.org/
Also keep in mind that all the encryption protonmail does for your email, while great, is dependent on the javascript being served to you; if their webserver is compromised, or they are "somehow" compelled to give someone access.. all bets are off.
I'd still recommend protonmail, use it myself, but know the advantages and disadvantages.
And more importantly, don't be lulled into a false sense of security.
Very valid points. There is an encrypted email option which only sends a link to the recipients who then need to decrypt in browser with password.
karnal is absolutely on the money here... smtp encryption is something of a crapshoot - STARTTLS downgrade attacks are pretty trivial due to backwards compatibility.
https://www.elie.net/blog/understanding-how-tls-downgrade-attacks-prevent-email-encryption
When it comes to email, to be absolutely certain you do have to go full tinfoil hat and install some PGP variant...