Outgoing mail is generally pushed from the sender to the final mail delivery agent (and possibly via intermediate mail servers) using Simple Mail Transfer Protocol. If the receiver uses a polling email delivery protocol, the final step from the last mail delivery agent to the client is done using a poll. Post Office Protocol (POP3) is an example of a polling email delivery protocol. At login and later at intervals, the mail user agent (client) polls the mail delivery agent (server) to see if there is new mail, and if so downloads it to a mailbox on the user's computer. Extending the "push" to the last delivery step is what distinguishes push email from polling email systems.
The reason that polling is often used for the last stage of mail delivery is that, although the server mail delivery agent would normally be permanently connected to the network, it does not necessarily know how to locate the client mail user agent, which may only be connected occasionally and also change network address quite often. For example, a user with a laptop on a Wi-Fi connection may be assigned different addresses from the network DHCP server periodically and have no persistent network name. When new mail arrives to the mail server, it does not know what address the client is currently assigned.
The Internet Message Access Protocol (IMAP) provides support for polling and notifications. When a client receives a notification from a server, the client may choose to fetch the new data from the server. This makes retrieval of new messages more flexible than a purely push system, because the client can choose whether to download new message data.
Although push email had existed in wired-based systems for many years, one of the first uses of the system with a portable, "always on" wireless device outside Asia was the BlackBerry service from Research In Motion. In Japan, "push email" has been standard in cell phones since 2000.
Apple's iPhone, iPad and iPod Touch support Hotmail push email. Until early 2013, they supported Gmail push email (via Google Sync) and Microsoft's Exchange ActiveSync platform, allowing them to synchronize email, calendars and contacts with mail servers such as Microsoft Exchange Server, Kopano, Zimbra, and Kerio Connect. Apple's iCloud service offers support for push email, contacts, and calendars, although as of the 24 February 2012, this has been temporarily disabled in Germany due to lawsuits. However, by setting up a new account using IMAP IDLE, push email is restored.
BlacMail, from Fifth C, is a push email solution that supports all major public email systems and targeted at mass-market consumer phones. The solution is unique in that a single client supports both SMS or GPRS as the message bearer, important attributes in the fastest developing mobile markets such as India.
Android's built-in Gmail client uses Google Cloud Messaging to push email for Gmail accounts set up to sync with the phone. Android also supports Microsoft Exchange accounts natively through its default mail application. When "Push" is configured, emails arriving into the Microsoft Exchange inbox are instantly pushed to the device. Calendar events sync both ways between Exchange and the device.