Hey damshaw,

I haven't done this before but have done alot of Xforms development as of late.  This can be done and you will need to hook into the BeforeSubmitPostedData/ AfterSubmitPostedDataEvent to check the answers and there you can decide whether to send the email or return.  Totally up to you and how you are going to handle your data.  These events are located by default in the global asax file. 

You will just have to loop the controls to retrieve your values

foreach (Control control in tempControls)
            {
                if (control is Input)
                {
                    var inputControl = (Input)control;
                 }
            }

   Hope this helps.