Error message when attempting to save an email message or attachment directly to a WebDrive mapped drive: “The message interface has returned an unknown error.”

To fix this error, disable a feature called “Robustified UNC” by using the Registry (regedit). Open regedit and navigate to the following location and make the change noted below:

  • Outlook 2007 Key
  • Outlook 2010 Key

Find the value named: DisableRobustifiedUNC 

Ensure the  value type is: REG_DWORD 

Make the value: 1

This will disable the Robustified UNC feature. You will want to restart your computer after making this registry change.

The “Robustified UNC” feature involves the security measures used when opening a file, especially a .tmp file.

Disabling the “Robustified UNC” feature may also fix issues with slow performance when opening/saving files that are located on a network share.