The protocol commonly used to send and receive emails is SMTP (Simple Mail Transfer Protocol) for sending messages and IMAP (Internet Message Access Protocol) or POP3 (Post Office Protocol) for receiving them. SMTP handles the transmission of outbound messages, while IMAP and POP3 manage the retrieval of incoming messages from the server to the user's device. These protocols work together to facilitate effective email communication, ensuring reliability and user access to their mailbox.