Post Office Protocol version 3 (POP3) is usually used by mail user agents (MUAs) to download messages from a remote mailbox. POP3 supports a single mailbox only, it does not support advanced multi-mailbox operations offered by alternatives such as IMAP.

The POP3 protocol uses a single TCP connection to give access to a single mailbox. It uses a simple command/response based approach, the client issues a command and a server can respond either positively or negatively.