You can validate the email address to make sure that it is in a valid format, but your ASP.NET application only handles sending the email to the SMTP server. You can't get an error if the email bounces back because the application never sees anything from the email after it successfully sends it to the server.
Life is like a box of chocolates. A cheap, thoughtless, perfunctory gift that nobody ever asks for. Unreturnable because all you get back is another box of chocolates. So you're stuck with this undefinable whipped mint crap that you mindlessly wolf down when there's nothing else left to eat. Sure, once in a while there's a Peanut Butter Cup or an English Toffee. But they're gone too fast and the taste is... fleeting. So you end up with nothing but broken bits filled with hardened jelly and teeth-shattering nuts. And if you're desperate enough to eat those, all you got left is an empty box... filled with useless brown paper wrappers.