Items from an Exchange account are stored in the Outlook cache. If this cache becomes corrupted, it may cause synchronization problems with the Exchange server. To solve this problem, you can empty the cache in Outlook so that Outlook can download all the items from your Exchange account again.
Caution: The following procedure deletes any information that is not synchronized with the Exchange server, including your contacts' mail certificate. Emptying the cache replaces the contents of the folder with the latest items from the Exchange server. Before emptying the cache, you may want to ensure that your Outlook information is backed up.
Make sure that your computer is connected to the Exchange server.
In the navigation pane, Ctrl+click or right-click the Exchange folder for which you want to empty the cache, and then click Properties.
On the General tab, click Empty Cache. After the folder is empty, Outlook automatically downloads the items from the Exchange server.