sendding WM_COPYDATA always return 0

im using WM_COPYDATA to send data to an other application, but the result from sendmessage is always 0.
How can i check if a message has been processed by an application if it always returns 0 ?
I know 100% sure the other program gets the message cause it show a messagebox.

receiving app

Code:

    case WM_COPYDATA:
        cdsPtr = (COPYDATASTRUCT*) lParam;
        switch(cdsPtr->dwData)
        {
        case (OPEN_CONNECTION):
            //MBOX("opened");
            return TRUE;
            break;
......

sending app

Code:

int result = SendMessage(sendHwnd,WM_COPYDATA,(WPARAM) hwnd,(LPARAM) &cps );