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STARTTLS Command

STARTTLS is an SMTP command used to upgrade a plain text connection to a secure, encrypted connection using Transport Layer Security (TLS).^[600-developer__smtp.md]

Mechanism

The command is utilized by Mail User Agents (MUA) or Mail Transfer Agents (MTA) after the initial connection to an SMTP server.^[600-developer__smtp.md] To initiate the security upgrade, the client first confirms the server's capabilities by sending the EHLO command.^[600-developer__smtp.md]

If the server supports Extended SMTP (ESMTP), it responds with a list of supported features.^[600-developer__smtp.md] If STARTTLS is included in this list, the client can then send the STARTTLS command to request a TLS handshake negotiation.^[600-developer__smtp.md]

  • [[EHLO]]

Sources