How to check email id in textbox exist or not in asp.net c#

To check for email valid:

string email = txtemail.Text;
Regex regex = new Regex(@"^([\w\.\-]+)@([\w\-]+)((\.(\w){2,3})+)$");
Match match = regex.Match(email);
if (match.Success)
    Response.Write(email + " is corrct");
else
    Response.Write(email + " is incorrct");

To check if the email actually exists, you would have to send an email and check if it bounces back.