The user authentication method for sending E‑mail from a client. The client is given permission to use the SMTP server by accessing the POP3 server before sending the E-mail.
SMTP‑AUTH (SMTP Authentication)
SMTP‑AUTH expands SMTP (the Internet E-mail sending protocol) to include an authentication method that ensures the true identity of the sender is known.
APOP (Authenticated Post Office Protocol)
APOP expands POP3 (the Internet receiving protocol) to include an authentication method that encrypts the password when the client receives E‑mail.
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