I have no personal experience with Proton Mail, but I do know that Thunderbird cannot interact directly with it. It (and other IMAP/SMTP email clients) require "Proton Mail Bridge", a program that acts as an intermediary between Thunderbird and the Internet. The basic idea seems to be that you set up Thunderbird to address Proton Mail Bridge, which in turn talks to the Proton Mail servers. Here's an article about it:. https://proton.me/support/imap-smtp-and-pop3-setup Some other info pages about the Mail Bridge: https://proton.me/support/mail