Try this. These were the settings for Outlook Mobile 5. Although I have a device using WM6 I no longer sync to an Exchange server, so I'm not sure if the process has changed.
1 Tap Start and select Programs.
2 At the Programs screen, tap ActiveSync.
3 At the ActiveSync screen, tap Menu and select Options.
4 At ActiveSync's Options screen, tap and hold on E-mail until the menu pops up. Select Settings.
5 At ActiveSync's E-mail Sync Options screen, you can set how much mail to include when your device synchronizes with the mail server. The default setting for item age is 3 days. Any message older than three days will not be copied to your device. If you want to sync older items, then tap the "Include the previous" pulldown and select a different value.