When starting ActiveSync for the first time, you are prompted to set up a synchronization relationship. This allows you to synchronize information with your device. Though your device can have sync relationships with two PCs, e-mail messages can be synchronized with only one PC.
See also
Overview of synchronization relationships
Connect to ActiveSync using a cable or a cradle
Connect to ActiveSync using infrared
Connect to ActiveSync using Bluetooth
Set up sync relationships with two PCs