Resetting dat-files

The most common dat-files that can be reset for troubleshooting are outcmd.dat and extend.dat. To reset them you can browse to their location and rename them to .old when Outlook is closed.

Note: Outlook 2010 and Outlook 2013 no longer use the the extend.dat and outcmd.dat files.

outcmd.dat

Keeps toolbar customizations. Resetting this will restore the Toolbar to default and recreates the icons that some add-ins might have installed.

• Windows XP
  C:\Documents and Settings\%username%\Application Data\Microsoft\Outlook\
• Windows Vista, Windows 7 and Windows 8
  C:\Users\%username%\AppData\Roaming\Microsoft\Outlook\

extend.dat

Keeps a registry cache of installed Exchange Client Extensions (ECE) add-ins and their status. Resetting this will force Outlook to retrieve the settings from the registry again and reset their status.

• Windows XP
  C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\
• Windows Vista, Windows 7 and Windows 8
  C:\Users\%username%\AppData\Local\Microsoft\Outlook\

You can paste the paths mentioned above in the Address Bar of Explorer to quickly open the location.

Note 1: To easily locate the folders for you, you can also use OutlookTools (free!)
Note 2: For an overview of the locations of other Outlook files and settings see Files and Settings guide for Outlook.