POP3
Post Office Protocol 3 — an older email retrieval protocol that downloads messages from a server to a local device, typically removing them from the server afterward.
POP3 (Post Office Protocol version 3, RFC 1939) is one of the earliest protocols for retrieving email. A POP3 client connects to the mail server, downloads new messages to the local device, and — by default — deletes them from the server. This download-and-delete model was designed for the era when users had a single device and limited server storage.
Because POP3 removes messages from the server, it effectively creates a local archive, but it does so on only one device. If you check your email on a second device using POP3, that device gets a different set of messages. This contrasts sharply with IMAP, which keeps everything on the server and syncs across all devices.
Some mail services still offer POP3 access alongside IMAP, and older client configurations may still use it. If you have accumulated a POP3-downloaded archive in an application like Outlook Express or Apple Mail, that archive is likely stored in MBOX or a similar format and can be opened in tools that support those formats.
Related terms
Internet Message Access Protocol — the standard protocol for accessing email stored on a server, keeping messages synchronized across multiple devices without downloading and deleting them.
Simple Mail Transfer Protocol — the standard protocol used to send and relay email messages between mail servers. It is used for outgoing mail only; reading email requires IMAP or POP3.