POP3 (Post Office Protocol version 3)

POP3, or Post Office Protocol version 3, is an application-layer protocol used by email clients to retrieve email messages from a remote server over a TCP/IP connection. It is the third version of the protocol and is designed to enable simple, standardized access to mailboxes for downloading messages to the user's local computer.

When using POP3, email messages are typically downloaded and stored on the user’s device, then deleted from the email server. This allows users to access their emails offline, which is one of the key benefits of POP3. However, because emails are removed from the server after they are downloaded, accessing emails from multiple devices can be problematic unless the email client is configured to leave a copy of the messages on the server.

POP3 operates on a "retrieve and delete" model and uses port 110 by default for non-encrypted connections, or port 995 for connections that use SSL/TLS encryption, providing a secure pathway for retrieving emails. The protocol includes commands that allow the email client to connect to the email server, retrieve the list of messages, download them, delete them from the server, and then disconnect.

While POP3 is widely used, it is generally considered less flexible compared to more modern protocols like IMAP (Internet Message Access Protocol), which allows multiple clients to manage the same mailbox, helping users to synchronize their email messages across various devices. Despite this, POP3 remains popular for its simplicity and efficiency in situations where single-device email access is sufficient.