关于邮件群发出现的问题各位大侠帮帮忙！！

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net.Mail;
using System.ServiceProcess;namespace MailManagerSystem
{
    public partial class SendForm : Form
    {
        //邮件附件全路径文件名
        private string[] strFilePaths = new string[5];
        //待发送目标邮件地址
        private string[] strMailAddress;
        //待发送目标邮件地址封装属性
        public string[] StrMailAddress
        {
          get { return strMailAddress; }
          set { strMailAddress = value; }
        }
        //构造函数
        public SendForm()
        {
            InitializeComponent();
        }
        //打开文件浏览对话框事件
        private void btnOpenFile_Click(object sender, EventArgs e)
        {
            //附件个数限制
            if (lbxFile.Items.Count >=4)
            {
                MessageBox.Show("附件添加最大个数不能超过5。");
                return;
            }
            //设置对话框初始属性
            openFile.InitialDirectory = "C:\\";
            openFile.Filter =
                "All Files (*.*)|*.*|HTML Files (*.htm;*.html)|*.htm|Microsoft Mail Documents (*.msg)|*.msg|Word Documents (*.doc)|*.doc|Excel Files(*.xl*)|*.xl*|Excel Worksheets (*.xls)|*.xls|Excel Charts (*.xlc)|*.xlc|PowerPoint Presentations (*.ppt)|*.ppt|Text Files (*.txt)|*.txt";
            openFile.FilterIndex = 1;            //打开文件对话框
            if (openFile.ShowDialog() == DialogResult.OK) 
            {
                //记录全路径文件名
                int iCount = lbxFile.Items.Count;
                strFilePaths[iCount] = openFile.FileName;                //获得文件名
                String[] strFileName = openFile.FileName.Split('\\');
                System.Array.Reverse(strFileName);
                lbxFile.Items.Add(strFileName[0]);
            }
        }
        // 发送邮件
        private void btnSend_Click(object sender, EventArgs e)
        {
            if (tbxMailFrom.Text == "")
            {
                MessageBox.Show("必须输入发信邮箱地址。", this.Text, MessageBoxButtons.OK, MessageBoxIcon.Warning);
                return;
            }
            if (tbxMailSubject.Text == "")
            {
                MessageBox.Show("主题不能为空！", this.Text, MessageBoxButtons.OK, MessageBoxIcon.Warning);
                return;
            }            pbSendMail.Value = 0;
            pbSendMail.Maximum = StrMailAddress.Length;
            //打开计时器
            timer.Start();
            //当计时器事件中的代码执行完成后，显示提示信息
            MessageBox.Show("邮件发送成功！", this.Text, MessageBoxButtons.OK, MessageBoxIcon.Warning);
        }
        // 计时器计数
        private void timer_Tick(object sender, EventArgs e)
        {
            //创建新邮件
            //参数分别用来初始化邮件发送和接收的地址
            MailMessage mail = new MailMessage(tbxMailFrom.Text, StrMailAddress[pbSendMail.Value]);
            //设置邮件主题
            mail.Subject = tbxMailSubject.Text;
            //设置邮件附件
            for (int i = 0; i<5; i++)
            {
                if (strFilePaths[i] != "" && strFilePaths[i] != null)
                {
                    mail.Attachments.Add(new Attachment(strFilePaths[i]));
                }
            }
            //设置邮件文本
            mail.Body = tbxMailText.Text;            try
            {
                //设置SMTP服务器地址
                SmtpClient client = new SmtpClient("smtp.163.com");
                //发送邮件
                client.Send(mail);
                ++pbSendMail.Value;
                if (pbSendMail.Value >= pbSendMail.Maximum)
                {
                    //邮件全部发送完毕后计时器停止
                    timer.Stop();
                    MessageBox.Show("邮件发送完毕。", this.Text, MessageBoxButtons.OK, MessageBoxIcon.Warning);
                }
            }
            catch (Exception exp)
            {
                timer.Stop();
                MessageBox.Show("邮件发送时发生以下错误：" + exp.Message, this.Text, MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }        private void groupBox1_Enter(object sender, EventArgs e)
        {
                    }        private void toolStripButton2_Click(object sender, EventArgs e)
        {
            this.colorDialog1.AllowFullOpen = true;
            this.colorDialog1.AnyColor = true;
            this.colorDialog1.FullOpen = true;
            this.colorDialog1.SolidColorOnly = false;
            this.colorDialog1.ShowDialog();
            this.tbxMailText.ForeColor = this.colorDialog1.Color;
        }        private void toolStripButton1_Click(object sender, EventArgs e)
        {
            if (fontDialog1.ShowDialog() == DialogResult.OK)
            {
                this.tbxMailText.Font = this.fontDialog1.Font;            } 
        }        
    }
}
出错为：邮件发送发生以下错误：不允许使用邮箱名称，服务器响应为： Authentication is required ,stmp10. Dscowlb7fqcwutpm4cfoaa--.11521s2 1278916885
各位大侠帮帮忙