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Deciphering the extend.dat Outlook error message

Frustrated admins have been dealing with extend.dat error messages since Outlook 97. Our expert sheds some light on this all-too-common Outlook error.

I just got the Outlook error message, “An extension failed to initialize. Cannot open file: extend.dat.” Why did...

this happen and what can I do about it?

The extend.dat file prevents Outlook from looking at registry settings each time it needs to access information for an add-on that will improve Outlook performance. Extend.dat is usually no bigger than 1 KB and is stored in a proprietary binary format that only Outlook can read. This file often gets corrupted, when it tries to create a new item or when a user exits the application.

The extend.dat error message is nothing new; I’ve seen reports of it as far back as Outlook 97. This tells me that either that the underlying mechanism Outlook uses hasn’t changed much since Microsoft first implemented it or it’s meant to be backwards compatible.

Microsoft did remove the extend.dat file from Outlook 2010; however, the file may still exist if you’re working with an Outlook 2010 client that was migrated from Outlook 2007 or earlier. In this scenario, the file won’t still be active.

Most often, I've seen the error message in this form:

An extension failed to initialize. Cannot open file: extend.dat. The file may not exist, you may not have permission to open it, or it may be open in another program.

Other times, the same error prompts this more cryptic message:

Could not complete the operation. One or more parameter values are not valid.

This version is far more frustrating because it’s unclear what operation caused the error.

If an extend.dat file has become corrupted without any warning, here are a few theories as to why this may have happened:

  • Third-party add-ons and extensions -- Third-party add-ons are one of the first things I troubleshoot as a cause of Outlook errors. A poorly written extension can also cause an error.
  • A disk error -- Although disks have become more reliable over the past 10 years, errors still happen. An improperly copied or restored installation of Outlook or the Outlook user profile may send an error message if extend.dat was not preserved correctly.
  • A previous Outlook crash corrupted the extend.dat file -- Outlook crashed and the extend.dat file was somehow mangled during the crash.

If your extend.dat file is corrupted, you should delete it. Depending on which Windows OS you’re running, you will find the extend.dat file in different locations. If you're running Windows Vista or Windows 7, you'll find the extend.dat file in the C:\Users\<username>\AppData\Local\Microsoft\Outlook folder. It is also accessible via the %UserProfile%\AppData\Local\Microsoft\Outlook folder.

In Windows XP, you can find the extend.dat file in the C:\Documents and Settings\<username>\Local Settings\Application Data\Microsoft\Outlook folder.

Because extend.dat is a cache file, deleting it will not damage your Outlook installation. The system will recreate file content the next time Outlook launches. However, before you delete the file, close Outlook completely. If you’re reluctant to completely delete the file, rename it something like extend.dat.broken.

Another potential problem
If deleting extend.dat doesn’t resolve the error, then the Outlook user profile may be corrupt, which is a much larger problem. There is a Microsoft KnowledgeBase article that describes how to create a new user profile for Outlook. However, this article raises another issue.

The KB article recommends that you create a new user profile first, instead of deleting it. By recreating the Outlook profile, you recreate many of your Outlook settings from scratch. In my opinion, you should only do this if it's absolutely clear that deleting extend.dat will not help.

About the author:
Serdar Yegulalp has been writing about computers and IT for more than 15 years for a variety of publications, including SearchWinIT.com, SearchExchange.com, InformationWeek and Windows magazine.

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