public void Process()
        {
            var recordCount = 1000;//每次取出郵件記錄數
            while (true)
            {
                using (var client = new Pop3Client())
                {
                    //1、建立連接，并進行身份認證
                    client.Connect(server, port, useSSL);
                    client.Authenticate(userName, pwd);

                    var messageCount = client.GetMessageCount(); // 郵箱中現有郵件數
                    if (messageCount > recordCount)
                    {
                        messageCount = recordCount;
                    }
                    if (messageCount < 1)
                    {
                        break;
                    }
                    var listAllMsg = new List<Message>(messageCount); //用于臨時保存取出的郵件

                    //2、取出郵件后填充至列表，每次最多recordCount封郵件
                    for (int i = 1; i <= messageCount; i++) //郵箱索引是基于1開始的，索引范圍: [1, messageCount]
                    {
                        listAllMsg.Add(client.GetMessage(i)); //取出郵件至列表
                    }

                    //3、遍歷并保存至客戶端，格式為.eml
                    foreach (var message in listAllMsg)
                    {
                        var emlInfo = new System.IO.FileInfo(string.Format("{0}.eml", Guid.NewGuid().ToString("n")));
                        message.SaveToFile(emlInfo);//保存郵件為.eml格式文件
                    }

                    //4、遍歷并刪除
                    int messageNumber = 1;
                    foreach (var message in listAllMsg)
                    {
                        client.DeleteMessage(messageNumber); //刪除郵件（本質上，在關閉連接前只是打上DELETE標簽，并沒有真正刪除）
                        messageNumber++;
                    }

                    //5、斷開連接，真正完成刪除
                    client.Disconnect();

                    if (messageCount < recordCount)
                    {
                        break;
                    }
                }
            }
        }

        public void DeleteMessage(int messageNumber)
        {
            AssertDisposed();

            ValidateMessageNumber(messageNumber);

            if (State != ConnectionState.Transaction)
                throw new InvalidUseException("You cannot delete any messages without authenticating yourself towards the server first");

            SendCommand("DELE " + messageNumber);
        }

        private void SendCommand(string command)
        {
            // Convert the command with CRLF afterwards as per RFC to a byte array which we can write
            byte[] commandBytes = Encoding.ASCII.GetBytes(command + "\r\n");

            // Write the command to the server
            OutputStream.Write(commandBytes, 0, commandBytes.Length);
            OutputStream.Flush(); // Flush the content as we now wait for a response

            // Read the response from the server. The response should be in ASCII
            LastServerResponse = StreamUtility.ReadLineAsAscii(InputStream);

            IsOkResponse(LastServerResponse);
        }

   /// <summary>
        /// This is the stream used to read off the server response to a command
        /// </summary>
        private Stream InputStream { get; set; }

        /// <summary>
        /// This is the stream used to write commands to the server
        /// </summary>
        private Stream OutputStream { get; set; }

      /// <summary>
        /// Connects to a remote POP3 server
        /// </summary>
        /// <param name="hostname">The <paramref name="hostname"/> of the POP3 server</param>
        /// <param name="port">The port of the POP3 server</param>
        /// <param name="useSsl">True if SSL should be used. False if plain TCP should be used.</param>
        /// <param name="receiveTimeout">Timeout in milliseconds before a socket should time out from reading. Set to 0 or -1 to specify infinite timeout.</param>
        /// <param name="sendTimeout">Timeout in milliseconds before a socket should time out from sending. Set to 0 or -1 to specify infinite timeout.</param>
        /// <param name="certificateValidator">If you want to validate the certificate in a SSL connection, pass a reference to your validator. Supply <see langword="null"/> if default should be used.</param>
        /// <exception cref="PopServerNotAvailableException">If the server did not send an OK message when a connection was established</exception>
        /// <exception cref="PopServerNotFoundException">If it was not possible to connect to the server</exception>
        /// <exception cref="ArgumentNullException">If <paramref name="hostname"/> is <see langword="null"/></exception>
        /// <exception cref="ArgumentOutOfRangeException">If port is not in the range [<see cref="IPEndPoint.MinPort"/>, <see cref="IPEndPoint.MaxPort"/> or if any of the timeouts is less than -1.</exception>
        public void Connect(string hostname, int port, bool useSsl, int receiveTimeout, int sendTimeout, RemoteCertificateValidationCallback certificateValidator)
        {
            AssertDisposed();

            if (hostname == null)
                throw new ArgumentNullException("hostname");

            if (hostname.Length == 0)
                throw new ArgumentException("hostname cannot be empty", "hostname");

            if (port > IPEndPoint.MaxPort || port < IPEndPoint.MinPort)
                throw new ArgumentOutOfRangeException("port");

            if (receiveTimeout < -1)
                throw new ArgumentOutOfRangeException("receiveTimeout");

            if (sendTimeout < -1)
                throw new ArgumentOutOfRangeException("sendTimeout");

            if (State != ConnectionState.Disconnected)
                throw new InvalidUseException("You cannot ask to connect to a POP3 server, when we are already connected to one. Disconnect first.");

            TcpClient clientSocket = new TcpClient();
            clientSocket.ReceiveTimeout = receiveTimeout;
            clientSocket.SendTimeout = sendTimeout;

            try
            {
                clientSocket.Connect(hostname, port);
            }
            catch (SocketException e)
            {
                // Close the socket - we are not connected, so no need to close stream underneath
                clientSocket.Close();

                DefaultLogger.Log.LogError("Connect(): " + e.Message);
                throw new PopServerNotFoundException("Server not found", e);
            }

            Stream stream;
            if (useSsl)
            {
                // If we want to use SSL, open a new SSLStream on top of the open TCP stream.
                // We also want to close the TCP stream when the SSL stream is closed
                // If a validator was passed to us, use it.
                SslStream sslStream;
                if (certificateValidator == null)
                {
                    sslStream = new SslStream(clientSocket.GetStream(), false);
                }
                else
                {
                    sslStream = new SslStream(clientSocket.GetStream(), false, certificateValidator);
                }
                sslStream.ReadTimeout = receiveTimeout;
                sslStream.WriteTimeout = sendTimeout;

                // Authenticate the server
                sslStream.AuthenticateAsClient(hostname);

                stream = sslStream;
            }
            else
            {
                // If we do not want to use SSL, use plain TCP
                stream = clientSocket.GetStream();
            }

            // Now do the connect with the same stream being used to read and write to
            Connect(stream, stream); //In/OutputStream屬性初始化
        }

        public Message GetMessage(int messageNumber)
        {
            AssertDisposed();

            ValidateMessageNumber(messageNumber);

            if (State != ConnectionState.Transaction)
                throw new InvalidUseException("Cannot fetch a message, when the user has not been authenticated yet");

            byte[] messageContent = GetMessageAsBytes(messageNumber);

            return new Message(messageContent);
        }

      if (askOnlyForHeaders)
            {
                // 0 is the number of lines of the message body to fetch, therefore it is set to zero to fetch only headers
                SendCommand("TOP " + messageNumber + " 0");
            }
            else
            {
                // Ask for the full message
                SendCommand("RETR " + messageNumber);
            }

    /// <summary>
        /// Tests a string to see if it is a "+OK" string.<br/>
        /// An "+OK" string should be returned by a compliant POP3
        /// server if the request could be served.<br/>
        /// <br/>
        /// The method does only check if it starts with "+OK".
        /// </summary>
        /// <param name="response">The string to examine</param>
        /// <exception cref="PopServerException">Thrown if server did not respond with "+OK" message</exception>
        private static void IsOkResponse(string response)
        {
            if (response == null)
                throw new PopServerException("The stream used to retrieve responses from was closed");

            if (response.StartsWith("+OK", StringComparison.OrdinalIgnoreCase))
                return;

            throw new PopServerException("The server did not respond with a +OK response. The response was: \"" + response + "\"");
        }

      /// <summary>
        /// Save this <see cref="Message"/> to a file.<br/>
        /// <br/>
        /// Can be loaded at a later time using the <see cref="LoadFromFile"/> method.
        /// </summary>
        /// <param name="file">The File location to save the <see cref="Message"/> to. Existent files will be overwritten.</param>
        /// <exception cref="ArgumentNullException">If <paramref name="file"/> is <see langword="null"/></exception>
        /// <exception>Other exceptions relevant to file saving might be thrown as well</exception>
        public void SaveToFile(FileInfo file)
        {
            if (file == null)
                throw new ArgumentNullException("file");

            File.WriteAllBytes(file.FullName, RawMessage);
        }