pop3s

POP3S refers to the use of the Post Office Protocol version 3 (POP3) over TLS or SSL to secure the connection between email clients and servers. It is not a distinct protocol; rather, it is POP3 transported within an encrypted TLS session. The common deployment uses implicit TLS, with clients connecting to the mail server on port 995 and performing a TLS handshake immediately before any POP3 commands. After the handshake, the server presents a certificate, and the client verifies it before exchanging POP3 commands such as USER, PASS, STAT, LIST, RETR, or DELE. All data, including credentials and messages, is transmitted over the encrypted channel.

POP3S contrasts with POP3 over STARTTLS, where the client connects on the standard POP3 port (110) and upgrades to TLS via the STLS command. In practice, POP3S is widely supported and is often easier to configure since the TLS layer is established upfront.

Security considerations include that POP3S protects credentials and messages in transit but does not provide end-to-end encryption or protection for data at rest on the server. Correct certificate validation and up-to-date cipher suites are important to prevent man-in-the-middle attacks. Some servers may support only STARTTLS or offer IMAP with TLS; in modern practice, many services favor IMAPS (IMAP over TLS) or other secure retrieval methods, while POP3S remains in use on systems that require POP3 compatibility.

See also: POP3, IMAP, STARTTLS, IMAPS.