since they own the OS on the PDA, they can optimize the performance with their own mail servers, with a proprietary protocol. Unless GOOG and YHOO adopt Exchange Servers, which is unlikely because of the cost, it's hard to duplicate.
There are other mail clients on PDA (Palm OS or WM 5.0) that work as a push-mail client, I tried all of them. Nothing comes close to the msft's technology ("AKU2"). The orignal post was not quite correct, DirectPush preceded AKU2 and typically refers to the older implementation that relies on using SMS msg to notify the client and then client picks up the msg. It was a battery drain.
AKU2 is super in battery performance. I'm a big fan of it.
