The file: directive is fine, but you need to find the 'real' name of the drive. As you say, different people can use different letters to map a network drive, so you must specify the real name.
Go to a command prompt and find the remote name the drive maps to (it is one of the 'net' commands - try 'net help' as a start); or look in Explorer - right-click the drive and the properties should give the full path name of the drive the letter is mapped to. Use that name rather than the Z: which is just a shortcut.
You can check it out by typing this name into explorer, and it should show your Z: drive.