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 no longer uses 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 and Windows 7
  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 and Windows 7
  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 this guide.