I have a user that has currently been unable to open tiff attachements. If she gets a tiff file any other way but through outlook she is able to open it fine.
The error that is thrown is,
Can't create file: FAX.TIF. Right-Click the folder you want to create the filein , and then click Properties on the shortcut menu to check your permissions on the folder.
This user gets maybe twenty of these through the fax server every day.