I am not familiar with 'voting buttons' in outlook, however, it just seems to be a "content" problem, not an email problem. It is kind of like asking how to send HTML via email. You just send HTML, there is no magic behind it.

All outlook probably does is construct this content for you. All you need to do is either figure out how to make the content yourself, or have outlook do it, then save it, and send copies via your c# application. All you might need to do is some search and replace on the emails if they contain specific information per customer.