Wikipedia
DirectPush
DirectPush is Microsoft's technology for receiving e-mail instantly on Windows Mobile 5.0, 6.0 and 6.1 enabled devices, from Microsoft Exchange Servers, Kerio Connect and Zarafa.
It provides response times similar to the push technology of RIM's BlackBerry service, but needs no special server upgrades other than having Exchange Server 2003 Service Pack 2. It also eliminates the need for devices to have a dedicated IP address, but requires "Always on" GPRS or 3G connection.