The IMAP extension that allows push notifications for new messages is IDLE. Defined in RFC 2177, IDLE lets an email client keep an IMAP connection open while telling the server it is ready to receive mailbox-change notifications.
Without IDLE, a client generally has to poll the server repeatedly: it sends a status or search command, waits for the response, and repeats the process. IDLE reduces that unnecessary traffic and allows new-message alerts to arrive with little delay. The server advertises support by including “IDLE” in its CAPABILITY response.
IDLE is often described as “push email,” but it is technically a long-lived IMAP command rather than a completely separate push-delivery network. The client must still maintain the connection and periodically exit and re-enter IDLE, because servers may impose connection time limits. It also differs from Push-IMAP, a separate proposal designed specifically for push-style mail delivery.
The feature is associated with IMAP4 and is widely supported by modern mail servers and clients, although exact notification speed depends on network conditions, server settings, and the client’s implementation.