What is POP3?

POP3, or Post Office Protocol version 3, is an application-layer protocol used by email clients to retrieve messages from a remote mail server. Defined in RFC 1939 (1996), it supersedes earlier versions POP and POP2. Unlike IMAP, which keeps messages on the server for synchronized access across devices, POP3 is designed for offline use: it downloads all new messages to the local client and, by default, removes them from the server. This behavior conserves server storage but makes it difficult to access the same messages from multiple computers or mobile devices.

The protocol operates over TCP, typically on port 110 for plaintext connections or port 995 for POP3S (POP3 over TLS/SSL). A session follows a simple three-state model: authorization (user sends credentials), transaction (client issues commands such as LIST, RETR, DELE to list and download messages), and update (server deletes marked messages and closes the connection). POP3 supports only basic mailbox operations; there is no concept of folders, flags, or server-side search. Extensions like TOP (retrieve headers and a limited number of body lines) and UIDL (unique identifier listing) add modest functionality, but the protocol remains minimal compared to IMAP.

POP3 was the dominant protocol for personal email in the 1990s and early 2000s, when users typically accessed email from a single desktop computer with limited server disk space. Its simplicity made it easy to implement in early email clients such as Eudora, Outlook Express, and Pine. Today, POP3 is still used in environments where offline access is preferred or server storage is constrained, but most modern email services and clients default to IMAP for multi-device synchronization. The protocol is defined in RFC 1939 and updated by RFC 2449 (POP3 Extension Mechanism) and RFC 2595 (TLS support).