When configuring ActiveSync, Windows Mobile devices by default only display messages from the last three days in the Inbox. To change this default behavior, you must edit the account properties of your ActiveSync profile.  

On your device, open your Exchange mailbox and navigate to Tools > Options. Or you may find the option in the ActiveSync profile configuration, then choose Configure Server Options.

The name of the option varies depending on the device, but most Windows Mobile devices have it under Download the Past.  Modify the number of days to All or to a specific time range, as desired.