Without encryption, plain text email messages can be easily intercepted and tampered with in-transit. Modern email servers, especially major webmail providers such as Gmail or Outlook.com, do typically exchange emails over SSL/TLS connections, reducing the likelihood of tampering, but this does not equate to zero-knowledge, end-to-end encryption. An email provider could be compelled by the authorities to decrypt and hand over the messages, either during the transmission process or when they land in a users’ inbox.
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