Outlook Secure temp folder

The Outlook Secure Temp folder is a super hidden folder, so, unless you know the full path (and it's randomly named folder, you're not finding it. And that's fun when you have a user that's trying to play the 100th copy of a file with the same file name (can you say "voicemail messages"?) and they get an error message.

To find the Secure Temp folder, you have to hop into the registry.

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

Where the number is the number of your version of Outlook. 15 is Office 2013. The registy entry will give you the file path of the Secure Temp folder, go to it and delete the contents (after you make sure they don't have any attachments open from Outlook they haven't saved elsewhere). 

After that, the countdown to the next 100th file starts!