Push messaging was brought into the mainstream by Research In Motion, which got started selling two way pagers. RIM gradually advanced its network with a bolt-on system that interfaces with corporate messaging servers (such as Exchange Server or IBM's Lotus Notes Domino server), grabs any updates from the server, and sends them to RIM's own servers at its Network Operations Center, which then relays the messages over the mobile network to BlackBerry devices. This works out well for RIM, as it gets to sell the BlackBerry devices, the BlackBerry Enterprise Servers that connect to each company's mail server, rather expensive client access licenses (CALs) for each user served by the BES, and then on top of that charge steep monthly messaging fees for every user served by its relay system.
In response to RIM's success, Microsoft advanced its own competing push messaging component to Exchange Server (somewhat confusingly called Exchange ActiveSync or EAS, although it bears little in common with the desktop ActiveSync tool), which pushes out updates to mobile devices directly. Microsoft is currently still in a distant runner up position behind BES, so it does not yet charge for CALs for using EAS, and does not bill companies per-user service fees to relay messages to their own employees. Of course, Microsoft does still require per-mailbox CALs for every user on Exchange, which results in making it an expensive solution once you start adding users.