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What is "extend.dat" used for?

jbishton asked
Medium Priority
Last Modified: 2012-08-13
I deleted Active Sync and now I get a message saying that "extend.dat" is missing.  Outlook seems to work fine otherwise.  How can I correct this problem?
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Did you remove activesync through add remove programs? If not you either need to re-install it then remove it, or look in the addins section of outlook and remove any refrences to activesync. If all else fails just uninstall Outlook and re-install it. You should be able to bakup your profiles in another directory, then restore them when complete.
Will SzymkowskiSenior Solution Architect
Most Valuable Expert 2015
Top Expert 2015

Hello there,

EXTEND.DAT is an internal cache file used by Outlook to store paths to all DLL files used for Outlook extensions.

Hope this helps
You may just need to refresh the extend.dat file, which is a binary file that caches the registry extension entries.

Follow these steps to refresh the Extend.dat file:

1. Quit all programs.  
2. On the Windows Taskbar, click Start and then Find and Files or Folders.  
3. In the Named box, type extend.dat.  
4. In the Look in list, click to select your hard drive.  
5. Click to select Include subfolders and click Find Now.  
6. In the Name list, right-click on the file Extend.dat and on the shortcut menu, click Rename to rename the file to Extend.old.  
7. On the File menu, click Close to exit Find File.  
8. Restart Outlook. Restarting Outlook recreates a new Extend.dat file.  

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