只需对增加相应标识即可：
// 使用SmtpMail对象发送邮件MailMessage mailObj = new MailMessage();

mailObj.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1"); //设置需要验证
mailObj.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", username); //用户名
mailObj.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", password); //密码


－－－－－－－－－－－－－－示例函数－－－－－－－－－－－－－－－
public void SendMail (string subject,string txt,string mailserver,string username,string password,string from,string sendto)
{
MailMessage mailObj = new MailMessage();

// 设置邮件的发送及接收地址
mailObj.From = from;
mailObj.To = sendto;

mailObj.Subject = subject;
mailObj.Body = txt;

// html格式的邮件
mailObj.BodyFormat = MailFormat.Html;


// 设置为高级优先权
mailObj.Priority = MailPriority.High;

// 为邮件添加附件
// 使用Attachments集合的Add方法创建mailattachment对象添加一个附件到邮件中
//mailObj.Attachments.Add(new MailAttachment("c:\\test.doc"));

// 使用SmtpMail对象发送邮件
mailObj.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1"); //设置需要验证
mailObj.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", username); //用户名
mailObj.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", password); //密码

SmtpMail.SmtpServer=mailserver;
SmtpMail.Send(mailObj);
}

//调用示例
private void Button1_Click(object sender, System.EventArgs e)
{
try
{

SendMail("意见建议",this.TextBox.Text,"mail.orientalwisdom.com","bx133@orientalwisdom.com","123456","bx133@orientalwisdom.com","bx133@orientalwisdom.com");
this.Response.Write("<script>window.alert('您的意见已经提交，感谢您的宝贵建议');</script> ");
this.Response.Write("<script>window.location='../index.aspx';</script>");
}
catch(System.Exception ee )
{
Session["error"]=ee.Message;
Response.BufferOutput=true;
Response.Redirect("errorpage.aspx");
}
}