引入命名空間

using System.Net.Mail;
using System.Net;

private static bool SendMail(string toAddress, string mailSubject, string mailBody)
    {
        MailAddress from = new MailAddress("xiachufeng@126.com", "xiachufeng"); //设置发件人信箱,及显示名字
        MailAddress to = new MailAddress(toAddress, "gangge");  //设置收件人信箱,及显示名字
        MailMessage oMail = new MailMessage(from, to);   //创建一个MailMessage对象
        oMail.Subject = mailSubject;      //邮件标题      
        oMail.Body = mailBody;       //邮件内容

oMail.Attachments.Add(new Attachment("D:\\project.rar")); //附件
        oMail.IsBodyHtml = true;            //指定邮件格式,支持HTML格式       
        oMail.BodyEncoding = System.Text.Encoding.GetEncoding("utf-8");//邮件采用的编码       
        oMail.Priority = MailPriority.High;//设置邮件的优先级为高
        //发送邮件服务器
        SmtpClient client = new SmtpClient();
        client.Host = "smtp.126.com";    //指定邮件服务器
        client.Credentials = new NetworkCredential("xiachufeng@126.com", "你的密码");//指定服务器邮件,及密码
        try
        {
            client.Send(oMail);  //发送邮件
        }
        catch (Exception eall)
        {
            string str = eall.Message;
            return false;
        }
        finally
        {

oMail.Attachments.Dispose(); 
            oMail.Dispose();       //释放资源
        }
        return true;
    }

/// <summary>
   /// 调用 发送邮件
   /// </summary>
   /// <param name="sender"></param>
   /// <param name="e"></param>
    protected void Button1_Click(object sender, EventArgs e)
    {

bool b = SendMail("duchenggang.accp@163.com", "你好", "哈哈哈，做个测试gang");
        if (b == true)
        {
            ClientScript.RegisterStartupScript(this.GetType(), "onclick", "<script>alert('ok')</script>");

}
        else
        {
            ClientScript.RegisterStartupScript(this.GetType(), "onclick", "<script>alert('error')</script>");

}
    }