Check the report ID that is being sent (when doing the sending, if there is another pop-up for the report, press Ctrl+Alt+F1 and on right side You should see the report ID). In Report Layout Selection You can check what kind of layout is used. You can try to switch it to Word, and then in Custom Report Layouts create Your design.
But if the report contains code calculated values and has some advanced logic in it, it might not be possible.