验证邮箱是否存在（部分验证，功能有限，经供参考）

初步测试，待后续优化：

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Net.Sockets;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace Mail.Lib
{
    public class MailUtilities
    {
        TcpClient tcpc;
        NetworkStream s;
        string strDomain;
        byte[] bb;
        int len;
        string read;
        string stringTosend;
        byte[] arrayToSend;
        bool flag;
        int count = 0;

        public string getMailServer(string strEmail, bool IsCheck)
        {
            strDomain = strEmail.Split('@')[1];
            ProcessStartInfo info = new ProcessStartInfo();   //指定启动进程时使用的一组值。
            info.UseShellExecute = false;
            info.RedirectStandardInput = true;
            info.RedirectStandardOutput = true;
            info.FileName = "nslookup";
            info.CreateNoWindow = true;
            info.Arguments = "-type=mx " + strDomain;
            Process ns = Process.Start(info);        //提供对本地和远程进程的访问并使您能够启动和停止本地系统进程。
            StreamReader sout = ns.StandardOutput;

            Regex reg = new Regex(@"mail exchanger = (?<mailServer>[^\s]+)");
            string strResponse = "";
            while ((strResponse = sout.ReadLine()) != null)
            {

                Match amatch = reg.Match(strResponse);   // Match  表示单个正则表达式匹配的结果。

                if (reg.Match(strResponse).Success)
                {
                    return amatch.Groups["mailServer"].Value;   //获取由正则表达式匹配的组的集合

                }
            }
            return null;
        }

        private void Connect(string mailServer)
        {
            try
            {
                tcpc.Connect(mailServer, 25);
                s = tcpc.GetStream();
                len = s.Read(bb, 0, bb.Length);
                read = Encoding.UTF8.GetString(bb);
                if (read.StartsWith("220") == true)
                    Console.WriteLine("连接服务器成功！" + "\r\n");
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }
        }

        private bool SendCommand(string command)
        {
            try
            {

                arrayToSend = Encoding.UTF8.GetBytes(command.ToCharArray());
                s.Write(arrayToSend, 0, arrayToSend.Length);
                len = s.Read(bb, 0, bb.Length);
                read = Encoding.UTF8.GetString(bb);
                Console.WriteLine("收到：" + read.Substring(0, len) + "\r\n");
            }
            catch (IOException e)
            {
                Console.WriteLine(e.ToString());
            }
            if (read.StartsWith("250"))
                return true;
            else
                return false;
        }

        public bool checkEmail(string mailAddress)
        {
            Regex reg = new Regex(@"\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*");

            if (!reg.IsMatch(mailAddress))
            {
                Console.WriteLine("Email地址形式上就不对" + "\r\n邮箱不存在！");
                return false;
            }//Email地址形式上就不对


            string mailServer = getMailServer(mailAddress, true);

            if (mailServer == null)
            {
                Console.WriteLine("邮件服务器不存在！" + "\r\n邮箱不存在！");
                return false;
                //邮件服务器探测错误
            }
            Console.WriteLine("解析出域名" + strDomain + "的mx记录：" + mailServer + "\r\n");
            tcpc = new TcpClient();      //为 TCP 网络服务提供客户端连接。
            tcpc.NoDelay = true;
            tcpc.ReceiveTimeout = 3000;
            tcpc.SendTimeout = 3000;
            bb = new byte[512];

            try
            {

                string strResponse = "";
                string strTestFrom = "haha@163.com";

                Console.WriteLine("连接服务器：" + mailServer + "\r\n");
                Connect(mailServer);

                string mailServer2 = getMailServer(strTestFrom, false);


                stringTosend = "helo " + mailServer2 + "\r\n";
                Console.WriteLine("发送：" + stringTosend);
                flag = SendCommand(stringTosend);

                if (flag == false)
                {
                    return false;
                }

                stringTosend = "mail from:<" + strTestFrom + ">" + "\r\n";
                Console.WriteLine("发送：" + stringTosend);
                flag = SendCommand(stringTosend);

                if (flag == false)
                {
                    return false;
                }


                stringTosend = "rcpt to:<" + mailAddress + ">" + "\r\n";
                Console.WriteLine("发送：" + stringTosend);
                flag = SendCommand(stringTosend);

                if (flag == true)
                {
                    Console.WriteLine("邮箱存在！");
                    return true;
                }
                else
                {
                    Console.WriteLine("用户不存在！" + "\r\n邮箱不存在！");
                    return false;
                }
            }
            catch (Exception ee)
            {
                Console.WriteLine(ee.ToString());   //发生错误或邮件服务器不可达
            }

            return false;
        }
    }
}