POP3

POP3, or Post Office Protocol version 3, is an Internet protocol used by email clients to retrieve messages from a mail server. It is designed to be simple and to allow offline access by downloading messages to a local device, with optional removal of those messages from the server after retrieval.

Operation and commands: Clients connect via TCP to the mail server. The protocol operates in three states: authorization, transaction, and update. After the client authenticates (for example with USER and PASS, or with alternative methods such as APOP or SASL), it issues commands to retrieve messages. Key commands include LIST (list messages and sizes), RETR (retrieve a full message), DELE (mark a message for deletion), UIDL (get unique identifiers for messages), TOP (retrieve headers and a portion of the body), NOOP, RSET, and QUIT. Messages marked for deletion are actually removed during the update state when the session ends.

Security and ports: POP3 was designed without encryption by default, so credentials and message contents are sent in plain text unless TLS is used. To protect data in transit, POP3 can run over TLS on port 995 (POP3S) or be upgraded to TLS via STARTTLS on port 110. Some servers also support stronger authentication methods such as APOP or SASL.

Comparison and usage: Compared with IMAP, POP3 is simpler and typically downloads messages to the client, often removing them from the server. It is well suited for offline access and archiving on a single device but offers limited server-side organization and search. POP3 remains supported by many servers and clients, though IMAP is generally preferred for multi-device access.