static bool mailsend = true;

  public async Task Send(object sender, EventArgs e)

         {

             SmtpClient smtp = new SmtpClient();//实例化一个SMPTClient对象

             smtp.EnableSsl = false;//不启用SSL加密

             smtp.Host = "00.000.00.000";//此处填写服务器IP

             smtp.Port = ;//端口固定为25

             //smtp.Credentials = new NetworkCredential("user@.com","password");//验证用户

             MailMessage msg = new MailMessage();//实例化一个Message对象

             msg.Priority = MailPriority.High;//邮件优先级

             msg.To.Add("user@foxmail.com");//收件人

            // msg.CC.Add("user@163.com");//抄送

             msg.Bcc.Add("user@qq.com");//密送

             string fileName = "";

             var na = Request.Files.AllKeys;//前端HTTP请求过来的文件

             foreach (var item in na)

             {

                 HttpPostedFileBase file = Request.Files[item] as HttpPostedFileBase;

                 fileName = Path.GetFullPath(file.FileName);

             }

             Attachment attach = new Attachment(fileName);//将文件路径付给Attachment的实例化对象

             ContentDisposition dispo = attach.ContentDisposition;//获取信息并读写附件

             dispo.CreationDate = System.IO.File.GetCreationTime(fileName);

             dispo.ModificationDate = System.IO.File.GetLastWriteTime(fileName);

             dispo.ReadDate = System.IO.File.GetLastAccessTime(fileName);

             msg.Attachments.Add(attach);//将附件加入邮件中

             msg.From = new MailAddress("SpadesQ@sea.com", "是你呀");//发件人 别名

             msg.Subject = "我学习了，你呢？";

             msg.SubjectEncoding = System.Text.Encoding.UTF8;

             msg.Body = "等下一个她";

             msg.BodyEncoding = System.Text.Encoding.UTF8;

             smtp.SendCompleted += new SendCompletedEventHandler(client_SendCompleted);

             string userState = "Task one";

             smtp.SendAsync(msg, userState);//使用异步发送 ，否则线程阻塞

             if (mailsend==false) //可在回调时修改mailsend值进行取消

             {

                 smtp.SendAsyncCancel();

             }

             Response.Write("已发出");

         }

          void client_SendCompleted(object sender, AsyncCompletedEventArgs e)

         {

             string token =(string)e.UserState;

             if (e.Cancelled)

             {

                 Response.Write(string.Format("{0} Send canceled.",token));

             }

             if (e.Error !=null)

             {

                 Response.Write(string.Format("[{0}] {1}", token, e.Error.ToString()));

             }

             else

             {

                 Response.Write("Message Send.");

             }

             mailsend = true;

         }