What protocol is commonly used by mobile devices to push email notifications from Microsoft Exchange?

The protocol commonly used by mobile devices to push email notifications from Microsoft Exchange is Exchange ActiveSync.

Exchange ActiveSync, commonly abbreviated EAS, is a Microsoft protocol for synchronizing email, contacts, calendars, tasks, and notes between an Exchange server and mobile devices. It communicates using XML over HTTP or HTTPS and was designed for intermittent, bandwidth-conscious mobile connections rather than traditional desktop mail access.

Its push capability is known as Direct Push. Instead of repeatedly downloading the entire mailbox, a device maintains a request that allows the server to signal when new information is available; the device then synchronizes the changes. This is why EAS is more than a simple mail-retrieval protocol. SMTP is primarily used to submit or relay outgoing mail, while POP3 and ordinary IMAP retrieve or synchronize mail but are not Microsoft’s Exchange mobile push protocol in this context.

The technology was originally branded AirSync and first appeared with Microsoft’s Mobile Information Server 2002. Direct Push was introduced with EAS 2.5, associated with Exchange Server 2003 SP2. EAS clients have been available on platforms including Windows Phone, Android, iOS, and BlackBerry 10.