In practice, email is pretty much all encrypted in transit these days (almost all SMTP and IMAP happen over SSL/TLS). You don’t need to worry about random third parties on the internet scanning your emails in transit.
Email, however, is not end-to-end encrypted: your own email provider (Gmail, your ISP, whatever) can see all your messages. Many actively scan your email to profile you. (This also applies to the email providers of the •recipients• of your emails.)
This is the problem Proton Mail claims to fix.
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