接收邮件时汉字出现乱码(不可解码)

最近用c#写了个接收邮件的客户端程序，
当邮件原文中存在未编码的汉字时，
接收到本地后，这些汉字就成了乱码****************************************************
下面是通过网页查看邮件原文时邮件头的部分内容Subject: (AD)填简历，测试你现在到底值多少钱？
MIME-Version: 1.0
Content-Type: text/html;
charset="gb2312"
Content-Transfer-Encoding: 8bit****************************************************
接收到本地后，“Subject”字段就成了下面的乱码，其它字段正常Subject: (AD)����������������ڵ���ֵ����Ǯ��****************************************************下面是接收邮件部分的c#代码        private string CRLF = "\r\n";
        private TcpClient client;
        private NetworkStream netStream;
        private StreamReader netStreamReader;        public string Receive(int mailNumber)
        {
            string strBack = WriteToStream("RETR " + mailNumber);
            if (strBack.StartsWith("+"))
            {
                StringBuilder strMailContent = new StringBuilder();
                do
                {
                    strBack = netStreamReader.ReadLine();
                    if (strBack != ".")
                    {
                        strMailContent.Append(strBack + CRLF); 
                    }
                }
                while (strBack != ".");
                return strMailContent.ToString();
            }
            else
            {                
            }
        }        private string WriteToStream(string command)
        {
              byte[] bytSend = Encoding.ASCII.GetBytes(command + CRLF);
            netStream.Write(bytSend, 0, bytSend.Length);
            string strBack = netStreamReader.ReadLine();            return strBack;
        }