If you need to change the number of selected months of email to synchronize with your local cached mode (.ost) file, do the following:
Open Outlook
Select File menu and click Account Settings > Account Settings
You should this window. Click once on your email address and select Change
At the next window, move the slider to increase the number of years to download email for the past:
Click the Next button.
Click Done on the next window and restart Outlook for the changes to take effect.