Outlook SecureTemp Folder by Sam C. Chan

November 5, 2013

Outlook Attachment Cache: Name Conflict Issues

Symptoms Summary

Suddenly, 1 or more specific user(s) on a system running Outlook mysteriously fail to open "seemingly" a certain type of attachments, and "seemingly" only from a certain source. What is known: "default application"  settings, as well as corruption of Outlook itself and/or the application for the intended attachment have all been ruled out.


This happens under certain circumstances, if the attachments in question have the same filename. It does not pertain the sender's platform. Standard behavior of Outlook 97/2000/2002(XP)/2003/2007/2010/2013:

  1. When an attachment is directly opened within a message (as opposed to saved first), a copy of the file is saved in the Outlook SecureTemp folder, so that antivirus can scan it.
  2. The cached copy will be deleted by Outlook,
    • if the message is closed (or moved away from reading pane), while the attachment is still opened (in its application), but...
    • if the attachment is closed first, the cached copy will remain, even after the message is subsequently closed!
  3. If there is a name conflict in saving a cached file, Outlook appends a suffix of (2), (3) and so on, until (99)
  4. On the 100th conflicting occasion, Outlook will fail to open such an attachment with the same name.

e.g.   photo.jpg    photo(2).jpg    photo(3).jpg ...  photo(99).jpg


  1. Avoid repeatedly (>100x) using duplicated filename for attachments
  2. Avoid the problematic closing sequence outlined above in #2
  3. Simply manually empty SecureTemp folder, which is located within the Temporary Internet Folder (TIF). Up until 2003, the Outlook SecureTemp folder name is OLK???? (4 random alphanumeric characters). From 2007 on, it is called Content.Outlook which contains a subfolder named with 8 random alphanumeric characters.

The location of the SecureTemp is stored in the registry and you can change the path to point to a new location, but be sure the new folder exists before changing it in the registry,  otherwise you will have red x for images. The OutlookSecureTempFolder value is at the following key for your version of Outlook.

  • 2013 HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Security
  • 2010 HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Security
  • 2007 HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Security
  • 2003 HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security
  • 2002 HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security
  • 2000 HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Outlook\Security
  • 97 HKEY_CURRENT_USER\Software\Microsoft\Office\8.0\Outlook\Security

Note: If the folder does not exist, the key will be reset to a new OLK* or Content.Outlook folder under Temporary Internet Files.

Hints for Actual Procedure

shell:cache (entered into W7 search box) will find Content.Outlook folder among other app temp folders, more will be displayed, if Explorer is in "show hidden" mode. If that's entered into address bar of Explorer, it'd display the content of TIF instead. Typical default locations (many variants, app + os version/edition-dependent):

  • C:\Users\%username%\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Outlook\<randomfoldername>
  • %localappdata%\Temporary Internet Files\Content.Outlook
  • C:\Documents and Settings\%username%\<randomfoldername>

See Also:

  • test files (set of 99 files) to aid in troubleshooting & re-creation/simulation
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