下面的code都是经过我验证过的，肯定可以用：

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.Mail;
public partial class send_email: System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

}
protected void Button1_Click(object sender, EventArgs e)
{

string filename = this.FileUpload1.PostedFile.FileName;
string sAttach = filename;

try
{
MailMessage msg = new MailMessage();
msg.From = "dzy@gmail.com";
msg.To = "dzy@yahoo.com.cn";
msg.Subject = "test email";
msg.Body = "can you receive this email?";
msg.Priority = MailPriority.High;
msg.BodyFormat = MailFormat.Html;

// Build an IList of mail attachments.
if (sAttach != "")
{
MailAttachment attachment = new MailAttachment(sAttach);
msg.Attachments.Add(attachment);
}

SmtpMail.SmtpServer = "smtp.gmail.com";

//1 代表使用 local smtp, 2 外部 smtp
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusing", 2);

//SMTP Server
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserver", "smtp.gmail.com");

//Server port, gmail is 465
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserverport", "465");

//Authentication method, ssl or not, Username and password for the SMTP Server
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", 1);

//cdoBasic 基本验证 gmail使用ssl验证
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpusessl", true);

//账号
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", "dzy");

//密码
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", "123456");

SmtpMail.Send(msg);
Response.Write("Successed!!");
}
catch (Exception ex)
{
throw ex;
}
}
}