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The contents of the message stay private, and they can sign in to ProtonMail to view it. They can even configure ProtonMail to send them a notification email whenever they receive a new secure message in ProtonMail. In order to communicate with someone securely, you might want to ask them to set up a ProtonMail account just for that purpose. You’ll know that an email has come from a ProtonMail account (and thus, that it’s been encrypted internally) when you see a purple padlock in the “From” field next to your contact’s email address. ProtonMail also allows you to use a private domain name with the service, so it’s possible to use internal encryption on non-ProtonMail domains, too. Domains that use and will use this high-level encryption.
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