用C#发送邮件的问题！！！谢谢

Question

发送的邮件一直在队列里发不出去，查看日志发现
向远程域 'qq.com' 传递邮件失败。错误消息为 'An SMTP protocol error occurred.
'。 引起此错误的 SMTP 动作为 'RCPT'。远程服务器 的响应为 '550 Mailbox not
C#代码是照书上写的
    protected void Button1_Click(object sender, EventArgs e)
    {
        MailAddress to_email = new MailAddress(TextBox1.Text);
        MailAddress form_email = new MailAddress(TextBox2.Text);
        MailMessage message = new MailMessage(form_email, to_email);
        message.Subject = TextBox3.Text;
        message.Body = TextBox4.Text;
        SmtpClient sc=new SmtpClient("127.0.0.1",25);
        sc.SendCompleted += new SendCompletedEventHandler(sc_SendCompleted);
        sc.Send(message);

Answer 1

//发送邮件的类
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Net.Mail;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;public class SendEmailAndMobileMessage
    {
        public SendEmailAndMobileMessage()
        {
            //
            // TODO: 在此处添加构造函数逻辑
            //
        }        /// <summary>
        /// 发送邮件
        /// </summary>
        /// <param name="strSmtpServer">发送邮件的服务器SMTP</param>
        /// <param name="strFrom">发送人邮件</param>
        /// <param name="strFromPass">发送人密码</param>
        /// <param name="strto">接收邮件</param>
        /// <param name="strSubject">主题</param>
        /// <param name="strBody">内容</param>
        public static void SendSMTPEMail(string strSmtpServer, string strFrom, string strFromPass, string strto, string strSubject, string strBody)
        {
            System.Net.Mail.SmtpClient client = new SmtpClient(strSmtpServer);
            client.UseDefaultCredentials = false;
            client.Credentials = new System.Net.NetworkCredential(strFrom, strFromPass);
            client.DeliveryMethod = SmtpDeliveryMethod.Network;            System.Net.Mail.MailMessage message = new MailMessage(strFrom, strto, strSubject, strBody);
            message.BodyEncoding = System.Text.Encoding.UTF8;
            message.IsBodyHtml = true;            client.Send(message);
        }    }//页面代码 using System.Net.Mail;
using System.Threading;private Thread objThread;
private ThreadStart objThreadStart;private void SendApplyMail{       
 SendEmailAndMobileMessage.SendSMTPEMail("smtp.xxxx.com.cn",
        "[email protected]",
        "xxxxx",
        strSendEmail,
         "xxxxxxx！",
         strbody);
}
protected void Button1_Click(object sender, EventArgs e)
    {
 objThreadStart = new ThreadStart(SendApplyMail);
                    objThread = new Thread(objThreadStart);
                    objThread.Start();
}