From a Seattle Times article about the MS and Palm relationship:
The advantage of Windows Mobile 5.0 as the operating system is its ability to easily communicate with a Windows computer through the popular Microsoft Exchange server.
Unlike with the BlackBerry, a company doesn’t have to purchase a new server for e-mail to go to devices.
Gates said he hopes that aspect will attract companies that don’t want to spend money or time on an additional server. He said that of the 130 million Microsoft Exchange users today, only 50 million have some mobile e-mail. A third of those are RIM BlackBerry users. [emphasis mine]
Those stats help put in context how important it is for MS to get on the handset.