Another benefit is increased privacy. Because Tor uses multiple layers of encryption, there is no way for someone monitoring a user's internet connection to tell whether he or she is accessing ProtonMail. On the other end, ProtonMail will not see the user's real IP address, so there will be no way to tie a visit to a real identity.
Tor itself provides multiple layers of encryption, but on top of that, the ProtonMail hidden service is also protected by HTTPS. The company has managed to obtain a certificate for its .onion address from DigiCert, even though the .onion TLD does not exist on the open Internet. DigiCert has issued such a certificate before for Facebook's Tor hidden service.
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