邮件发送老出现问题，请高手指点

//程序如下

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Web.Util;
using System.Web.Mail;

namespace _51aspxMail
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                format.Items.Add(new ListItem("文本", "0"));
                format.Items.Add(new ListItem("HTML", "1"));
                format.Items[0].Selected = true;

                fromMail.Text = "suhui0312@yahoo.com.cn"; //发送方邮件
                fromMail.Enabled = false;
            }
        }

        private bool SendMail(string fromMail, string toMail, string ccMail, string bccMail, string subject, string body, string sendMode)
        {
            try
            {
                MailMessage myMail = new MailMessage();
                myMail.From = fromMail;
                myMail.To = fromMail;
                myMail.Cc = ccMail;
                myMail.Bcc = bccMail;
                myMail.Subject = subject;
                myMail.Body = body;
                myMail.BodyFormat = sendMode == "0" ? MailFormat.Text : MailFormat.Html;

                //附件
                string ServerFileName = "";
                if (this.upfile.PostedFile.ContentLength != 0)
                {
                    string upFileName = this.upfile.PostedFile.FileName;
                    string[] strTemp = upFileName.Split('.');
                    string upFileExp = strTemp[strTemp.Length - 1].ToString();
                    ServerFileName = Server.MapPath(DateTime.Now.ToString("yyyyMMddhhmmss") + "." + upFileExp);
                    this.upfile.PostedFile.SaveAs(ServerFileName);
                    myMail.Attachments.Add(new MailAttachment(ServerFileName));
                }

                myMail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", 1);
                myMail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", "suhui0312@yahoo.com.cn"); //发送方邮件帐户
                myMail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", "03125074625"); //发送方邮件密码

                SmtpMail.SmtpServer = "smtp.mail.yahoo.com.cn";
                SmtpMail.Send(myMail);

                return true;
            }
            catch
            {
                return false;
            }
        }

        protected void send_Click(object obj, EventArgs e)
        {
           try
          {
            bool flag = SendMail(fromMail.Text, toMail.Text, ccMail.Text, bccMail.Text, subject.Text, body.Text, format.SelectedValue);
            if (flag == true)
            {
                Response.Write("<script>alert('发送成功!');</script>");
            }
            else
            {
                Response.Write("<script>alert('发送失败!');</script>");
            }
          }
            catch(Exception ex)
{   ///跳转到异常错误处理页面
Response.Write(ex.Message.Replace("<br>","").Replace("\n",""));
}

        }
    }
}

//运行后老是发送失败 --------------------编程问答-------------------- 分少了点哈哈
帮UP --------------------编程问答-------------------- 各位高手帮忙看看，主要是什么问题

补充：.NET技术 ,  ASP.NET