Change how much e-mail to keep offline

Click File > Account Settings > Account Settings.

Select your account, and then click Change.

Under Offline Settings, move the slider to the desired amount of time, such as All, 12 months, 6 months, 3 months, or 1 month.

For example, if you move the slider to 3 months, only mail from the past three months will be synchronized from the Exchange server. Items older than this time reside only in your mailbox on the server. You can change this setting at any time.

By default, Outlook configures e-mail accounts to only synchronize 12 months of your email.