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SMTPS
Port 465 was initially used for the Secure SMTP (SMTPS) protocol, providing encrypted email transmission via SSL. Despite its early deprecation in favor of STARTTLS on port 587, it has seen a resurgence in use for secured email submissions. This port helps facilitate confidential communication between email clients and servers, ensuring message privacy over potentially insecure networks..
Originally, port 465 was assigned for SMTPS, which used SSL to secure SMTP transmissions. SSL negotiation begins immediately upon connection, encrypting the entire session from start to finish. This helped to prevent interception or tampering of emails during transit.
However, the IANA revoked the official assignment of port 465 for SMTPS, recommending instead the use of STARTTLS on port 587 to upgrade plaintext SMTP connections to encrypted ones dynamically. Despite this, some mail service providers continued to support port 465 as a convenient means of encrypted submission.
Today, port 465 is widely recognized in practice, offering implicit TLS encryption for SMTP transactions. Clients that connect to this port typically expect an encrypted channel right from connection initiation, differing from the opportunistic nature of STARTTLS on other ports.