using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.Mail;

namespace MailSender
{
    public partial class Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            MailMessage objMailMessage;
            MailAttachment objMailAttachment;

            // 创建一个附件对象
            objMailAttachment = new MailAttachment("C:\\1.xml");//发送邮件的附件

            // 创建邮件消息
            objMailMessage = new MailMessage();
            objMailMessage.From = "mytest110@sina.com";//源邮件地址
            objMailMessage.To = "********@qq.com";//目的邮件地址
            objMailMessage.Subject = "邮件发送标题：你好";//发送邮件的标题
            objMailMessage.Body = "邮件发送标内容：测试一下是否发送成功！";//发送邮件的内容
            objMailMessage.Attachments.Add(objMailAttachment);//将附件附加到邮件消息对象中

            //接着利用sina的SMTP来发送邮件，需要使用Microsoft .NET Framework SDK v1.1和它以上的版本
            //基本权限
            objMailMessage.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1");

            //用户名
            objMailMessage.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", "mytest110");

            //密码
            objMailMessage.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", "******");

            //如果没有上述三行代码，则出现如下错误提示：服务器拒绝了一个或多个收件人地址。服务器响应为:530 Authentication required

            //SMTP地址
            SmtpMail.SmtpServer = "smtp.sina.com";

            // 开始发送邮件
            // 在发送之前，去新浪邮箱里开启POP/SMTP设置    邮箱设置->账户->POP/SMTP设置->开启
            // 否则会报错误0x80040217. The server response was not available
            SmtpMail.Send(objMailMessage);
        }
    }
}

------解决方案--------------------
C# code

/// <summary>
    /// 邮件发送
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>