When you open attachments from the email message before saving to the hard drive, Outlook places copies in the SecureTemp folder. This folder is a hidden folder under Temporary Internet files.

Background: When you open attachments on email directly from Outlook, a copy is written to a temporary folder on your hard drive so your the virus scanner can check it before it is opened. The folder Outlook writes this temp file to is under the Temporary Internet Files folder where IE writes pages to when you surf the internet. This offers more security as only your user account and the computer administrator account can view the files under this folder. The only way to avoid "losing" attachments in this well hidden folder is to save them to your hard drive, before opening. Few people do this (myself included).

These files are supposed to be deleted when you close the opened attachment, however this only happens if the email message the attachment arrived on remains open - this is either an opened message in a new window or in the reading pane. When the message is closed (or you wait to preview another message) after the attachment is closed, the temporary file is deleted from the SecureTemp folder. If you close the open message or display another Outlook item in the reading pane but leave the attachment open, the temp file is not deleted when you close the attachment.

When this folder gets "full" you may have problems opening email attachments or embedded images won't display. If you open image attachments in Windows Photo or Image viewer and click next, you may see old images you thought you deleted. Delete the files in the folder to fix. See clean the SecureTempFolder automatically for VBA code to clear the folder, either every time you close Outlook or only when you want to make sure the folder is empty.

Outlook 2016

The user-hive registry keys do not work with Outlook 2016 (Outlook overrides it with the default location), however adding it to the Policies path will work. However, because Outlook 2016 does a better job of cleaning up the SecureTemp folder, attachments you open may be deleted from the folder except when messages are closed while their attachments are open.

HKEY_CURRENT_USER\Software\Policies\Microsoft\office\16.0\outlook\Security
String (REG_SZ): OutlookSecureTempFolder
Value: path to folder

To open the folder where the SecureTemp folder Outlook is using is stored, type or paste the following into the address bar of File Explorer and press Enter.

C:\Users\%username%\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Outlook

%localappdata%\Temporary Internet Files\Content.Outlook

SecureTemp Location for Outlook 2010 & 2013

Beginning with Outlook 2010, Outlook does a better job at cleaning up the secure temp files and it's possible you will not find the secure temp folder or files in the secure temp folder.

To quickly jump to the Temporary Internet Folder (TIF) in Windows 7 and above, type shell:cache in the Start menu's Search field and press Enter. This will open Windows Explorer to the TIF file. Outlook's SecureTemp folder should be visible (Content.Outlook or OLK*). If not, go to Windows Explorer's Organize, Folder and Search Options menu. On the View tab, enable the option to show Hidden Files and Folders.

In Vista, Windows 7/8/10 and newer, Outlook's secure temp folder is at C:\Users\username\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Outlook\8A0VMD3A, where 8A0VMD3A can be any random characters.
Type or paste this shortcut into the address bar of File Explorer to open the location:

%localappdata%\Temporary Internet Files\Content.Outlook

You can copy and paste the command found in the correct textbox (above) into the address bar of Windows Explorer then press Enter to jump to the SecureTemp folder, where you'll see the subfolder with the random character name. Using this method does not require you to change the hidden files and folder setting.

Outlook 2007

Outlook 2007 uses C:\Documents and Settings\username\Local Settings\Temporary Internet Files\Content.Outlook\8A0VMD3A, where 8A0VMD3A can be any random characters.

To jump to this folder, type shell:cache in the address bar of File Explorer. If you have the option set to Show hidden files and folders, you'll see the Content.Outlook folder, if not, type Content.Outlook at the end of the path.

In Vista and newer, the Temp Internet folder is at:
C:\Users\username\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Outlook

%localappdata%\Temporary Internet Files\Content.Outlook

The shortcut for Outlook 2007 files on Windows XP is:

%USERPROFILE%\Local Settings\Temporary Internet Files\Content.Outlook

You can copy and paste the command found in the correct textbox (above) into the address bar of Windows Explorer then press Enter to jump to the SecureTemp folder, where you'll see the subfolder with the random character name. Using this method does not require you to change the hidden files and folder setting.

Find or move SecureTemp using the registry

The location of the Securetemp is stored in the registry and you can change the path to point to a new location (which is easier to access and clear), but be sure the new folder exists before changing it in the registry. The OutlookSecureTempFolder string value is stored in the following key for your version of Outlook. If the path in the path in the registry is not valid, you will have red x's for images.

Outlook 2013

HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Security

Outlook 2010

HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Security

Outlook 2007

HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\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.