邮件无法发送到 SMTP 服务器。传输错误代码为 0x80040217。服务器响应为 not available

using System;
using System.Data;
using System.Configuration;
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 CDO;
using System.Web.Mail;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        //12976095
    }
    /// <summary>
    /// 发送邮件
    /// </summary>
    /// <param name="email">要发送的邮件地址</param>
    /// <param name="bodys">邮件内容</param>
    /// <param name="Titles">邮件标题</param>
    public void Emailto(string email, string bodys, string Titles)
    {
        //smtp服务器
        string SmtpServer = ConfigurationManager.AppSettings["RandVServer"];
        //smtp验证所需用户名
        string Username = ConfigurationManager.AppSettings["RandVSender"];
        //密码
        string UserPass = ConfigurationManager.AppSettings["RandVPass"];

        //配置
        CDO.Configuration conf = new ConfigurationClass();
        conf.Fields[CdoConfiguration.cdoSendUsingMethod].Value = CdoSendUsing.cdoSendUsingPort;
        conf.Fields[CdoConfiguration.cdoSMTPServer].Value = SmtpServer;
        conf.Fields[CdoConfiguration.cdoSMTPAuthenticate].Value = CdoProtocolsAuthentication.cdoBasic;
        conf.Fields[CdoConfiguration.cdoSendUserName].Value = Username;
        conf.Fields[CdoConfiguration.cdoSendPassword].Value = UserPass;
        conf.Fields[CdoConfiguration.cdoSMTPServerPort].Value = "25";
        conf.Fields.Update();

        //邮件发送
        MessageClass msg = new MessageClass();
        msg.Configuration = conf;
        msg.To = email;
        msg.From = Username;
        msg.Subject = Titles;
        msg.HTMLBody = "<html><body>" + bodys + "<br>" + "<a target='_blank' href='Http://www.randv.com'>" + "欢迎您加入共合网(http://www.randv.com)" + "</a>" + "</body></html>";
        msg.BodyPart.Charset = "gb2312";
        msg.HTMLBodyPart.Charset = "gb2312";
        msg.Send();
        msg = null;
    }
    protected void send_Click(object sender, EventArgs e)
    {
        string body = mailbody.Text.Trim();
        string mail = "kjun163@163.com";
        string title = "好啊";
        Emailto(mail,body,title);
    }
}
--------------------编程问答-------------------- 我顶  顶顶顶顶 --------------------编程问答-------------------- 我试了很多方法都发不过去，此方法行吗？我都没信心再试了 --------------------编程问答-------------------- 我还是试了一次，不行，没有using cdo命名空间啊 --------------------编程问答-------------------- 我也快不行了，被这东西搞得 --------------------编程问答-------------------- 唉每次试了各种方法都失败！ --------------------编程问答-------------------- - - 安得维 --------------------编程问答-------------------- 一样纠结中 --------------------编程问答-------------------- 参考：http://www.cnblogs.com/insus/articles/1689279.html --------------------编程问答-------------------- 1、dns解析是不是对
2、目前大多数邮件服务器不接受动态ip
3、CDO测试下来对安全认证支持不好 --------------------编程问答-------------------- 还是用System.Net.Mail吧，我也试了很多次System.Web.Mail，还是不行。
不知道是不是System.Web.Mail过时的原因。

补充：.NET技术 ,  ASP.NET