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Back up and create a new Outlook Exchange Cache file (.ost) 
If you are unable to synchronize your mailbox, calendar, or contacts with data on the server this may indicate your Outlook cache needs to be re-created.  You can verify that data on the server is correct either by using Outlook Web Access or a mobile device.  The recommended action in these instances would be to rebuild your Outlook cache and resynchronize your Exchange data.  Your Outlook cache is stored in an .ost file and is set by default for improved performance.   This article details the process of setting aside your Outlook cache to build a new one on your computer. 
Note:  

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Turn Off Cached Exchange Mode 
1. Click the Start Menu -> Control Panel -> Mail 
2. Click the button labeled Show Profiles 
3. Highlight the profile you are using for Exchange email 
4. Click the Properties button. 
5. Click the Email Accounts button 
6. Highlight the Microsoft Exchange account and click the Change button. 
7. Uncheck the box that says “Use “Use Cached Exchange Mode” Mode” 
8. Click the Next button 
9. Click Finish. 

Rename the OST files on your hard drive 
1. Click the Start Menu to Search files and folders (in XP, there is a button for search, for Vista and Windows 7, use the search bar.) 
2. Enter the term “.ost” to search for and locate all .ost files on your computer. 
3. Rename all of the .ost files that your search locates to something meaningful (e.g. change outlook.ost to backup1-outlook.ost).  Bear in mind that this is your cached data and when you re-open Outlook, this file will be recreated by downloading your mailbox data from the server to a new .ost file that will be created by Outlook automatically. 

NOTE: Alternatively, you may find this file in the default location listed below: 
Windows 7: C:\Users\username\AppData\Local\Microsoft\Outlook 

Turn On Cached Exchange Mode 
1. Click the Start Menu > Control Panel > Mail 
2. Click the button labeled Show Profiles 
3. Highlight the profile you are using for Exchange email 
4. Click the Properties button. 
5. Click the Email Accounts button 
6. Highlight the Microsoft Exchange account and click the Change button. 
7. Check the box that says “Use Cached Exchange Mode” 
8. Click the Next button 
9. Click Finish. 

Launch Outlook 
Once Outlook is restarted, all of your mailbox data including email, calendars, tasks and notes will download to your computer again.  This process may take a while to complete depending on the size of your mailbox, but you may create and send new items while this process happens.  You can watch the progress in the lower right corner of the Outlook application window.

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