POP3 is a protocol that allows email clients to obtain email messages that are stored on a remote server. It is an application-level protocol in the OSI model. The stored email messages can be downloaded and read by the local computer. The POP3 protocol can be used to download these messages from the email server.

POP3 is designed to receive emails, not to send them; it allows users with intermittent or very slow connections (such as modem connections) to download their emails while they have a connection and later check them when they are disconnected. It should be mentioned that most mail clients include the option to leave messages on the server so that a client using POP3 connects, obtains all messages, stores them on the user's computer as new messages, removes them from the server, and finally disconnects.

The following diagram shows the client and server communicating through the POP3 protocol:

The client establishes a connection to the server on TCP port 110. They then send their username and password to access the mailbox. Once the connection has been established, the user can obtain the email messages individually.