//new 出 客服端的 smtp
SmtpClient client = new SmtpClient(); 
client.Host = "smtp.163.com";
//需要发件人的账号 密码
client.Credentials = new System.Net.NetworkCredential("邮箱名", "密码");

------解决方案--------------------
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Net.Mail;
using System.Net.Mime;

   /// <summary> 
   /// 发送邮件的类 
   /// </summary> 
public class SendMail
{
   private MailMessage mailMessage;
   private SmtpClient smtpClient;
   private string password;//发件人密码 
   /**/
   /// <summary> 
   /// 处审核后类的实例 
   /// </summary> 
   /// <param name="To">收件人地址</param> 
   /// <param name="From">发件人地址</param> 
   /// <param name="Body">邮件正文</param> 
   /// <param name="Title">邮件的主题</param> 
   /// <param name="Password">发件人密码</param> 
   public SendMail(string To, string From, string Body, string Title, string Password)
   {
       mailMessage = new MailMessage();
       mailMessage.To.Add(To);
       mailMessage.From = new System.Net.Mail.MailAddress(From);
       mailMessage.Subject = Title;
       mailMessage.Body = Body;
       mailMessage.IsBodyHtml = true;
       mailMessage.BodyEncoding = System.Text.Encoding.UTF8;
       mailMessage.Priority = System.Net.Mail.MailPriority.Normal;
       this.password = Password;
   }
   /**/
   /// <summary> 
   /// 添加附件 
   /// </summary> 
   public void Attachments(string Path)
   {
       string[] path = Path.Split(',');
       Attachment data;
       ContentDisposition disposition;
       for (int i = 0; i < path.Length; i++)
       {
           data = new Attachment(path[i], MediaTypeNames.Application.Octet);//实例化附件 
           disposition = data.ContentDisposition;
           disposition.CreationDate = System.IO.File.GetCreationTime(path[i]);//获取附件的创建日期 
           disposition.ModificationDate = System.IO.File.GetLastWriteTime(path[i]);// 获取附件的修改日期 
           disposition.ReadDate = System.IO.File.GetLastAccessTime(path[i]);//获取附件的读取日期 
           mailMessage.Attachments.Add(data);//添加到附件中 
       }
   }
   /**/
   /// <summary> 
   /// 异步发送邮件 
   /// </summary> 
   /// <param name="CompletedMethod"></param> 
   public void SendAsync(SendCompletedEventHandler CompletedMethod)
   {
       if (mailMessage != null)
       {
           smtpClient = new SmtpClient();
           smtpClient.Credentials = new System.Net.NetworkCredential(mailMessage.From.Address, password);//设置发件人身份的票据 
           smtpClient.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
           smtpClient.Host = "smtp." + mailMessage.From.Host;
           smtpClient.SendCompl