Ultimately, all “zero knowledge encryption” for emails is based on trust. As 99.9% of emails are not PGP-encrypted, they are sent unencrypted (just with transit encryption) between email providers. When you receive an email, it’s unencrypted and you have to trust your provider that they don’t make a secret copy before saving it with encryption in your “zero knowledge” inbox. Same when you send an email - they have to send it without encryption if you send it to a “normal” recipient like Gmail or Outlook.
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