http://danj3tfcicjkzz4gj66efqfydhu4txjibwkvrylvjytsfjlv3eu47nqd.onion/index.php?dest=articles/email.xhtml
According to researchers, ProtonMail's encryption contains serious shortcomings. At the end of this report, I also link to an article detailing the issues with in-browser encryption in general. Mail clients are not supported except, again, through a paid feature called Protonmail Bridge .