You can't check if an email address exists (in the sense that any message sent to it can be read) in any way other than to send a message to it and see if you get a reply.
You can however check if there is a valid email address in the textbox:
public static Regex regex = new Regex(
"([a-zA-Z0-9_\\-\\.]+)@((\\[[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1"+
",3}\\.)|(([a-zA-Z0-9\\-]+\\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})",
RegexOptions.IgnoreCase
| RegexOptions.CultureInvariant
| RegexOptions.IgnorePatternWhitespace
| RegexOptions.Compiled
);
Match m = regex.Match(myTextBox.Text);
if (m.Success)
{
}
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