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POP3, or Post Office Protocol version 3, is an email retrieval protocol that downloads email messages from a mail server to a single, local device. By default, after the emails are downloaded, they are deleted from the server, which frees up storage space. This process provides users with offline access to their messages.

How POP3 works

The process for retrieving mail via POP3 is a simple, four-step sequence:

Connect: The email client (e.g., Microsoft Outlook or Apple Mail) connects to the mail server using a Transmission Control Protocol (TCP) connection. The default, unencrypted port is 110, while the secure, encrypted version (POP3S) uses port 995.

Authenticate: The client authenticates with the server by providing a username and password.

Retrieve and delete: The client downloads all new email messages from the server. Once the download is complete, the client typically deletes the original messages from the server. Some modern clients offer the option to leave a copy on the server, but this is not the default behavior.

Disconnect: The client closes the connection to the server.