Push e-mail

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Push e-mail is used to describe e-mail systems that provide an "always-on" capability, in which new e-mail is instantly and actively transferred (pushed) as it arrives by the mail delivery agent (MDA) (commonly called mail server) to the mail user agent (MUA), also called the email client. Most of today's clients are smartphones.

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[edit] Push e-mail vs. traditional e-mail

Traditional e-mail access over dial-up connections was and still is "pull" based: At login and later in intervals, the Mail User Agent (email reader) polls the Mail Delivery Agent (server) to see if there is new mail, and if so downloads it to a mailbox in the user's home directory. However, mail has always been pushed from the origin to the final Mail Delivery Agent. Extending this push to the last delivery step is what distinguishes push e-mail from traditional e-mail systems.

The reason that pull is the usual method for the last stage of mail delivery is that while the server Mail Delivery Agent would normally be permanently connected to the network, the client Mail User Agent is likely to only occasionally be connected. For example, a user with a laptop on a dial-up link might only connect his machine to the network a few times each day, and so his machine must be the one that initiates the mail transfer. If the server tries to initiate the connection, it is unable to find the client machine if the client is not available to receive the mail.

Post Office Protocol (POP3) is a popular example of a pull based mail delivery protocol.

The Internet Message Access Protocol (IMAP) provides both polling support and the ability to monitor for notifications - this means that a client can be made aware almost instantly of the existence of the new message, but explicitly fetches new message data. This makes retrieval of the message marginally slower by a matter of a round-trip time, but this is typically only a fraction of a second. Efficiency can exceed a purely-push system because the client can choose to download full messages only as the user requests them.

One of the first systems to provide "always on" notification of mail, and therefore a "push" system, was Blitzmail, developed by Dartmouth College starting in 1988. Source code to the system was later released to the public.

[edit] Push e-mail for mobile users

Although Push e-mail had existed in wired-based systems for many years, one of the first uses of the system with a portable, "always on" wireless device was the BlackBerry service from Research In Motion.

BlackBerry uses wireless Mail User Agent devices and a BlackBerry Enterprise Server (BES) attached to a traditional e-mail system. The BES monitors the e-mail server, and when it sees new mail for a BlackBerry user, it retrieves (pulls) a copy and then pushes it to the BlackBerry handheld device over the wireless network.

BlackBerry became very popular, in part because it offers remote users "instant" e-mail experience; new mails appear on the device as soon as they arrive, without the need for any user intervention. The handheld becomes a mobile, dynamically updating, copy of the user's mailbox.

As a result of the success of BlackBerry, other manufacturers have developed push e-mail systems for other handheld devices, such as Symbian based mobile phones or Windows Mobile 5.0 devices.

Apple's upcoming iPhone will also feature push mail, but this will be limited to personal Yahoo! email.

[edit] Using traditional e-mail to simulate push e-mail

Traditional mobile mail clients using the IMAP protocol may poll for new mail at frequent intervals, without downloading the mail to the client, thus providing a similar user experience as push e-mail.

IMAP in fact allows many notifications to be sent at any time, but not message data. The IDLE command is often used to signal the ability of a client to process notifications sent outside of a command running, which effectively provides a user experience identical to push.

[edit] Protocols used for push e-mail

In contrast to traditional e-mail, most of the protocols used in popular current systems are proprietary; for example, BlackBerry uses its own private protocols developed by RIM. Both the Push-IMAP standard and parts of the SyncML standards are attempting to develop more open solutions.

The Lemonade Profile is an IETF modification to IMAP and SMTP to make them more suited to the demands of mobile email, it relies on the existing IDLE command to provide instant email notification on the client device. IDLE is not strictly push-email although it does provide the same user experience.

[edit] Products implementing push e-mail

[edit] See also

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