Windows Explored

Everyday Windows Desktop Support, Advanced Troubleshooting & Other OS Tidbits

Outlook .dat Files

Posted by William Diaz on February 4, 2011


Knowing what the Outlook 2003 .dat file do can often assist’s you with troubleshooting Outlook issues. The primary .dat files are:

  • Extend.dat – C:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Outlook. Contains a cache of installed Exchange Client Extensions add-ins. Deleting this will tell Outlook to retrieve these settings from the registry and recreate the extend.dat file. These settings are stored in HKLM\Software\Microsoft\Exchange\Client\Extensions:
    OutlookExRegDat
  • Outcmd.dat – C:\Documents and Settings\user\Application Data\Microsoft\Outlook. Toolbar customizations are stored here, including those made by add-ins. A normal outcmd.dat is 10-20kb; un-tested, poorly written add-ins that populate or create toolbars can bloat1 or corrupt this file and are a common culprit behind slow Outlook start-up times2. Delete\rename this file when troubleshooting delays.
  • Frmcache.dat – C:\Documents and Settings\user\Local Settings\Application Data\Microsoft\FORMS. Outlook forms opened by the user are stored on the local hard disk in this cache file. Each time the form is accessed, Outlook goes to this file first to see if it is there, then compares it against the form stored in the forms library (Exchange) to see if the one in the library has been modified. If it has, it updates the cached form with the modified one, otherwise it loads the form from the cache. Examples of custom forms are “While You Were Out” and the “NYTonerRequest” form. Add-ins also use forms, e.g. Enterprise Vault. When troubleshooting issues opening forms, clear the forms cache from Outlook and in extreme cases you may need to delete this file along with any other file\folders in the FORMs folder.

Quick Lab: Open Extend.dat with Notepad or Notepad++ and examine. Now select all, delete, and save. Open Outlook and it will error. Note the add-ins in Outlook (from the add-in tab) are blank. Outlook does not attempt to recreate this file when it exist. It only reads it. Outlook will only create the file from the registry if it is not detected when Outlook starts.


1I have seen a few instances where outcmd.dat exploded to several hundred kb and added several minutes to Outlook startup. If you see this, don’t assume that deleting the outcmd.dat is a long term solution until you have eliminated the possibility of an add-in modifying the toolbar and recreating the problem.

2Of course, other factors can contribute to startup delays, e.g. lots of connections to document management databases (e.g. FileSite), several large psts, and a large ost.

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