SmtpClient smtp = new SmtpClient();
smtp.EnableSsl = false;
smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
smtp.Host = "smtp.163.com";
smtp.Credentials = new NetworkCredential("三角猫@163.com", "这是密码");

MailMessage mm = new MailMessage();
mm.From = new MailAddress("三角猫@163.com", "三角猫", Encoding.GetEncoding(936));
mm.To.Add("三角猫@gmail.com");

mm.SubjectEncoding = Encoding.GetEncoding(936);
mm.Subject = "三角猫发的测试邮件，呵呵";

mm.BodyEncoding = Encoding.GetEncoding(936);

////普通文本邮件内容，如果对方的收件客户端不支持HTML，这是必需的
string plainTextBody = "如果你邮件客户端不支持HTML格式，或者你切换到“普通文本”视图，将看到此内容";
mm.AlternateViews.Add(AlternateView.CreateAlternateViewFromString(plainTextBody, null, "text/plain"));

////HTML格式邮件的内容
string htmlBodyContent = "如果你的看到<b>这个</b>， 说明你是在以 <span style="color:red">HTML</span> 格式查看邮件<br><br>";
htmlBodyContent += "<a href="http://www.zu14.cn/">真有意思网</a> <img src="cid:weblogo">";   //注意此处嵌入的图片资源
AlternateView htmlBody = AlternateView.CreateAlternateViewFromString(htmlBodyContent, null, "text/html");

////处理嵌入图片
LinkedResource lrImage = new LinkedResource(@"d:\blogo.gif", "image/gif");
lrImage.ContentId = "weblogo"; //此处的ContentId 对应 htmlBodyContent 内容中的 cid:  ，如果设置不正确，将不会显示图片
htmlBody.LinkedResources.Add(lrImage);

mm.AlternateViews.Add(htmlBody);

////要求回执的标志
mm.Headers.Add("Disposition-Notification-To", "接收回执的邮箱@163.com");

////自定义邮件头
mm.Headers.Add("X-Website", "http://www.zu14.cn/");

////针对 LOTUS DOMINO SERVER，插入回执头
mm.Headers.Add("ReturnReceipt", "1");

mm.Priority = MailPriority.Normal; //优先级
mm.ReplyTo = new MailAddress("回复邮件的接收地址@yahoo.com.cn", "我自己", Encoding.GetEncoding(936));

////如果发送失败，SMTP 服务器将发送 失败邮件告诉我
mm.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;

////异步发送完成时的处理事件
smtp.SendCompleted += new SendCompletedEventHandler(smtp_SendCompleted);

////开始异步发送
smtp.SendAsync(mm, null);

       void smtp_SendCompleted(object sender, AsyncCompletedEventArgs e)
        {
            if (e.Cancelled)
            {
                MessageBox.Show("发送被取消");
            }
            else
            {
                if (e.Error == null)
                {
                    MessageBox.Show("发送成功");
                }
                else
                {
                    MessageBox.Show("发送失败： " + e.Error.Message);
                }
            }
        }