使用C#傳送電子郵件，使用微軟自帶的類庫，主要有一下類：

    1、微軟封裝好的MailMessage類：主要處理髮送郵件的內容（如：收發人地址、標題、主體、圖片等等）
    2、微軟封裝好的SmtpClient類：主要處理用smtp方式傳送此郵件的配置資訊（如：郵件伺服器、傳送埠號、驗證方式等等）
    3、SmtpClient主要進行了三層的封裝：Socket --> TcpClient --> SmtpClient

程式碼如下：

System.Text.Encoding _Encoding = System.Text.Encoding.UTF8;

        string content = "Send Mail";
        string sendaddress = "
 
[email protected]"; //郵件傳送人
        string password = "[email protected]";            //郵件傳送人密碼
        string usePassword = "true";
        int port = 25;      //埠
        string isSSL = "false";                  //是否使用
        string smtp = "smtp.qq.com";//smtp 伺服器
        string receiver = "[email protected]
 
";

        MailAddress from = new MailAddress(sendaddress, "傳送人名稱", _Encoding);
        MailMessage message = new MailMessage();
        message.From = from;
        message.To.Add(new MailAddress(receiver));
        message.Subject = "Send Mail";
        message.Body = content;
        message.BodyEncoding = _Encoding;
        message.IsBodyHtml = true;
        message.SubjectEncoding = _Encoding;
        message.Priority = MailPriority.High;
        //上傳附件
        string attachFilePath = @"download\config.xml;download\程式碼整潔之道.pdf";
        if (attachFilePath != null && attachFilePath.Length > 0)
        {
            string path = System.Web.HttpContext.Current.Server.MapPath("~/");
            string[] attachPathArr = attachFilePath.Split(';');
            for (int i = 0; i < attachPathArr.Length; i++)
            {
                attachPathArr[i] = path + "\\" + attachPathArr[i];
                Response.Write(attachPathArr[i]);
                Attachment attachment = new Attachment(attachPathArr[i]);
                message.Attachments.Add(attachment);
            }
        }

        SmtpClient client = new SmtpClient(smtp);
        if (!Convert.ToBoolean(usePassword))  //是否使用密碼
        {
            client.Credentials = CredentialCache.DefaultNetworkCredentials;
        }
        else
        {
            client.Credentials = new NetworkCredential(sendaddress, password);
        }

        client.DeliveryMethod = SmtpDeliveryMethod.Network;
        if (Convert.ToBoolean(isSSL, CultureInfo.CurrentCulture))    //是否使用SSL協議
        {
            client.EnableSsl = true;
        }
        else
        {
            client.EnableSsl = false;
        }
        client.Port = Convert.ToInt32(port);
        client.Send(message);