C＃编程发送邮件

一、SMTP协议简介1、  客户端通过服务器的25端口建立TCP/IP连接服务器端： 220 server.com Simple Mail Transfer Service Ready2、  客户端使用“HELO”命令标识发件人客户端：HELO server.com服务器端：250 server.com3、  客户端发送MAIL命令，服务器端以OK作为响应表明准备接收客户端： MAIL FROM: <[email protected]>服务器端： 250 OK4、  客户端发送RCPT命令标识收件人，服务器端回应是否愿意为收件人接受邮件客户端：RCPT TO: <[email protected]>服务器端：250 OK5、  协商结束后用命令DATA发送发送邮件客户端：DATA服务器端：354 Start mail input: end with <CRLF>.<CRLF>6、  客户端以.表示结束输入内容一起发送出去 客户端：Subject:  <CRLF>内容<CRLF>.<CRLF>7、客户端用QUIT命令退出。               客户端：QUIT服务器端：250 server.com closing transmission channel 二、SMTP客户端程序代码：1、  用VS.NET新建一个C#  WINDOWS应用程序项目，命名为SMTP。2、  在窗体上添加控件：一个按钮：设置属性name->btnsend；text->发送六个标签：分别提示服务器地址、发件人、收件人、主题、内容以及发送记录五个文本框：服务器地址txtsmtp、发件人txtfrom、收件人txtto、主题txtsub、内容txtmsg对应相应的标签放置。              一个列表框：lstlog 3、在程序开头添加以下名字空间：       using System.Data;using System.Net;using System.Net.Sockets;using System.IO;          4、在 btnsend_Click 函数中添加代码       private void btnsend_Click(object sender, System.EventArgs e){                 // 将鼠标形状改为漏斗状              Cursor cr = Cursor.Current;              Cursor.Current = Cursors.WaitCursor;              //定义变量              string Data;              byte[] szData;              string CRLF = "\r\n";              //创建与服务器25端口的连接              TcpClient SmtpServ = new TcpClient(txtsmtp.Text,25);                            lstlog.Items.Clear();                               //显示服务器初始信息                   NetworkStream NetStrm = SmtpServ.GetStream();                   StreamReader  RdStrm= new StreamReader(SmtpServ.GetStream());                   lstlog.Items.Add(RdStrm.ReadLine());                   //标志发件人                   Data = "HELO server " + CRLF;                                     szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());                   NetStrm.Write(szData,0,szData.Length);                   lstlog.Items.Add(RdStrm.ReadLine());                   //标志发件人                   Data = "MAIL FROM: " + "<" + txtfrom.Text + ">" + CRLF;                   szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());                   NetStrm.Write(szData,0,szData.Length);                  lstlog.Items.Add(RdStrm.ReadLine(                   // 标志收件人                   Data = "RCPT TO: " + "<" + txtto.Text + ">" + CRLF;                   szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());                   NetStrm.Write(szData,0,szData.Length);                   lstlog.Items.Add(RdStrm.ReadLine());                   //准备发送内容                   Data = "DATA " + CRLF;                   szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());                   NetStrm.Write(szData,0,szData.Length);                   lstlog.Items.Add(RdStrm.ReadLine());                                  //发送主题                   Data = "SUBJECT: " + txtsub.Text + CRLF ;                   //发送内容                   Data = Data+ txtmsg.Text + CRLF ;                   //结束发送                   Data = Data+ "." + CRLF;                   szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());                   NetStrm.Write(szData,0,szData.Length);                   lstlog.Items.Add(RdStrm.ReadLine());                                  //退出                   Data = "QUIT " + CRLF;                   szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());                   NetStrm.Write(szData,0,szData.Length);                   lstlog.Items.Add(RdStrm.ReadLine());                                  //关闭连接                   NetStrm.Close();                   RdStrm.Close();                   lstlog.Items.Add("连接关闭");                   lstlog.Items.Add("发送成功");                    //将鼠标恢复箭头状                   Cursor.Current = cr;              }
请问这个里说的服务器地址指的是什么。
举个例子说说~谢谢！