By default, Outlook Mobile will synchronize three days worth of email. You actually have the option to see much more. I like to have about a week’s worth of email stored in my Inbox.

To change this for Exchange, go to Start>Programs>ActiveSync. Then, go Menu>Options. Select Email and hit Settings. The top field lets you specify how far to go back. This works in all versions of Windows Mobile.

For IMAP or POP accounts, open the messaging app, go Menu>Tools> Options. Then tap the name of the account you want to change.

After that, tap “Send/Receive Schedule” (on some phones older than WM 6.1, it will go to an email account setup wizard in which you have to go through by tapping “next” in order to find the schedule dialog). Finally, you’ll see the “Download messages” pop up menu with choices for how many days worth of messages you want to download.