SmtpClient smtp                      

 //如果需要认证，则用下面的方式
smtp.Credentials = new NetworkCredential("user", "pwd");

------解决方案--------------------

using System;
using System.IO;
using System.Net;
using System.Text;
using System.Net.Mail;
using System.Net.Mime;
using System.Windows.Forms;
using System.Text.RegularExpressions;

namespace SendEmail
{
   public partial class EmailSender : Form
   {
       string regStr =string.Empty;
       string from = string.Empty;
       string to = string.Empty;
       string body = string.Empty;
       const string FROMEMAIL = "wmglove@sina.com.cn";
       const string FROMPWD = "不给你说";
       string pwd = string.Empty;
       public EmailSender()
       {
           InitializeComponent();
           regStr = @"^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$";
           cmbBox.SelectedIndex = 0;
       }

       private void btnSender_Click(object sender, EventArgs e)
       {
           to = txtBoxRecv.Text.Trim();
           body = txtBoxBody.Text.Trim();
           if (to.Length == 0) return;
           if (body.Length == 0) return;
           if (!Regex.IsMatch(to, regStr))
           {
               MessageBox.Show("接收的电子邮件的格式不正确!", "提示", MessageBoxButtons.OK, MessageBoxIcon.Error);
               return;
           }
           from = txtBoxSender.Text.Trim();
           pwd = txtBoxPwd.Text.Trim();
           if (from.Length == 0)
           {
               SendEmail(FROMEMAIL, FROMPWD, to, FROMEMAIL, body, "smtp.sina.com.cn");
           }
           else
           {
               if (pwd.Length == 0)
               {
                   MessageBox.Show("电子邮件的密码不能为空!", "提示", MessageBoxButtons.OK, MessageBoxIcon.Error);
                   return;
               }
               if (Regex.IsMatch(from, regStr))
               {
                   switch (cmbBox.SelectedItem.ToString())
                   {
                       case "QQ.com":
                           SendEmail(from, pwd, to, from, body, "smtp.qq.com.cn");
                           break;
                       case "163.com":
                           SendEmail(from, pwd, to, from, body, "smtp.163.com.cn");
                           break;
                       case "126.com":
                           SendEmail(from, pwd, to, from, body, "smtp.126.com.cn");
                           break;
                       case "sina.com":
                           SendEmail(from, pwd, to, from, body, "smtp.sina.com.cn");
                           break;
                       default:
                           break;
                   }
               }
               else
               {
                   MessageBox.Show("发送的电子邮件的格式不正确!", "提示", MessageBoxButtons.OK, MessageBoxIcon.Error);
                   return;
               }
           }
       }

       private void SendEmail(string from, string fromPwd, string to, string subject, string body,string emailType)
       {
           string attFile = txtBoxFile.Text.Trim();
           ContentDisposition cd;
           MailAddress addrFrom = new MailAddress(from, "from");
           MailAddre