To reduce the effect of the Outlook offline data file (.ost), the default number of months that are configured for your profile varies by the size of your hard disk.
If you have to change the number of selected months of email to synchronize with your cached mode .ost file, follow these steps:
On the File tab, click Account Settings, and then click Account Settings.
On the E-mail tab, double-click your Microsoft Exchange account.
In the Change Account dialog box, drag the Mail to keep offline slider to the desired number of months or to All to synchronize all email messages.
Click OK when you're prompted to restart Outlook to complete the configuration change.