using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Net.Sockets;
using System.IO;
using System.Threading;
using System.Security.Cryptography;

NetworkStream stream = Client.GetStream();

结果编译错误 ： The name 'Client' does not exist in the current context

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Threading;
using System.Net;
using System.Net.Sockets;
using System.IO;
using System.Text;
using System.Data.SqlClient;
这样能做哦。你的有不有？

还是不行的啊，我的部分代码如下 ：
using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Net.Sockets;
using System.IO;
using System.Threading;

namespace SMTP_Custom
{
partial class Form1 : Form
{
//默认端口为 25 :
public Form1()
{
InitializeComponent();
textBox1.Text = "smtp.263.net";
textBox2.Text = "25";
textBox3.Text = "(试验邮件)";
}

//Main()
[STAThread]
static void Main()
{
Application.Run(new Form());
}

//Send()
private void send()
{
string come = null;
string check = null;
NetworkStream stream = client.GetStream();
string hostName = Dns.GetHostName();
WriteToNetStream(ref stream, "HElO" + hostName);
come = ReadFromNetStream(ref stream);
//......

你 在BAIDU里输入 这个 ：Client.GetStream();
好大一堆答案哦。