The term “POP3S” typically refers to POP3 over SSL/TLS, which is a secure variant of the Post Office Protocol version 3 (POP3). Here’s a brief history of POP3S:

1. Introduction of SSL/TLS: In the mid-1990s, SSL (Secure Sockets Layer) was developed by Netscape Communications as a security protocol for encrypted communication over the internet. SSL provided authentication and encryption to protect data transmitted between clients and servers.
2. SSL/TLS with POP3: To secure POP3 communication, SSL/TLS was integrated with the protocol, resulting in POP3 over SSL/TLS, often referred to as POP3S. This variant of POP3 allows for the encryption of the entire POP3 session, ensuring the confidentiality and integrity of email retrieval.
3. SSL to TLS transition: SSL was eventually succeeded by TLS (Transport Layer Security) as the standardized security protocol. Although the term “POP3S” still persists, it typically refers to the usage of TLS rather than SSL.
4. Operation of POP3S: POP3S operates similarly to POP3, but with the added layer of SSL/TLS encryption. The client establishes a secure connection with the POP3S server using SSL/TLS, encrypting the communication between the client and server. The standard port for POP3S is usually 995.
5. Popularity of STARTTLS: Over time, an alternative approach called STARTTLS gained popularity for securing POP3 communication. STARTTLS allows for the negotiation of a secure TLS connection within the standard POP3 protocol, eliminating the need for a separate port like in the case of POP3S.

As a result of the industry’s shift towards STARTTLS, the usage of POP3S has become less common. Many email clients and servers now prioritize STARTTLS as the preferred method for securing POP3 communication. It offers greater flexibility and compatibility since it can work on the same port as non-encrypted POP3, allowing for seamless transitioning between encrypted and non-encrypted connections.