static bool ValidateEmailAccount(string server, int port, string userName, string password, out string ErrorMessage)
        {
            ErrorMessage = "";

            //create a tcp connection
            TcpClient _server = new TcpClient(server, port);
            
            //prepare 
            NetworkStream netStream = _server.GetStream();
            StreamReader reader = new StreamReader(_server.GetStream());

            if (!reader.ReadLine().Contains("+OK"))
            {
                //失敗
                ErrorMessage = "server連結失敗";
                return false;
            }

            string data;
            byte[] charData;
            string CRLF = "\r\n";

            //login
            data = "USER " + userName + CRLF;
            charData = System.Text.Encoding.ASCII.GetBytes(data);
            netStream.Write(charData, 0, charData.Length);
            if (!reader.ReadLine().Contains("+OK"))
            {
                //賬戶錯誤
                ErrorMessage = "賬戶錯誤";
                return false;
            }
            data = "PASS " + password + CRLF;
            charData = System.Text.Encoding.ASCII.GetBytes(data);
            netStream.Write(charData, 0, charData.Length);
            if (!reader.ReadLine().Contains("+OK"))
            {
                //密碼錯誤
                ErrorMessage = "密碼錯誤";
                return false;
            }
            return true;
        }
 

 呼叫

            string errorMessage;
            bool isContains = ValidateEmailAccount("pop3.163.com", 110, "[email protected]", "************", out errorMessage);

            Console.WriteLine(isContains ? "使用者存在" : errorMessage);