how to check any email address, any mobilenumber is exists or not in multiline textbox

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)
   {
   // OK
   }