You have posted this in the Tinycam software forum; you might have better luck posting in the Dahua camera forum instead.
For the record I performed a SET URL like yours with an Amcrest IP2M-841 camera (Dahua OEM) and it accepted the email recipient, verified by opening the camera's webGUI. However, I could not include the authentication at the front of the URL (it returned an error), I entered it when prompted by the webGUI. I then performed a GET and received the new, correct results as a response in my browser.
Do you receive a error message from the camera's webGUI when you entered the SET command URL?