Validar e-mail.

ProgramEmail.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace EmailValido
{
    class Program
    {
        static void Main(string[] args)
        {

            Console.Write("Email: ");
            string email = Console.ReadLine();

            if (EhEmailValido(email))
                Console.WriteLine("Valido");
            else
                Console.WriteLine("Invalido");

            Console.ReadKey();
        }

        static bool EhEmailValido(string email)
        {
            return Regex.IsMatch(email, @"^(?("")("".+?""@)|(([0-9a-zA-Z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-zA-Z])@))" + @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-zA-Z][-\w]*[0-9a-zA-Z]\.)+[a-zA-Z]{2,6}))$");            
        }
    }
}