The popular email protocol used to retrieve messages from a mail server is termed Post Office Protocol (POP) or its more advanced version, IMAP (Internet Message Access Protocol). POP allows clients to download emails from the server and typically removes them from the server upon retrieval, making it suitable for offline access. IMAP, on the other hand, enables users to access their emails while keeping them stored on the server, providing synchronization across multiple devices. These protocols are essential in facilitating email communication in various applications and devices.