public static bool SendEmail(EmailInfo mailInfo)
        {
            try
            {
                MailMessage mailMsg = new MailMessage();
                mailMsg.To.Add(new MailAddress(mailInfo.ToAddress));
                mailMsg.From = new MailAddress(mailInfo.FromAddress, mailInfo.DisplayName);
                mailMsg.Subject 
 
= mailInfo.Subject;
                mailMsg.AlternateViews.Add(AlternateView.CreateAlternateViewFromString(mailInfo.Body, Encoding.UTF8, MediaTypeNames.Text.Html));
                SmtpClient smtpClient = new SmtpClient(mailInfo.SmtpHost, mailInfo.SmtpPort);
                System.Net.NetworkCredential credentials 
 
= new System.Net.NetworkCredential(mailInfo.SmtpUserName, mailInfo.SmtpPassword);
                smtpClient.Credentials = credentials;
                if (!string.IsNullOrEmpty(mailInfo?.filename) && mailInfo?.stream != null)
                {
                    mailMsg.Attachments.Add(
 
new Attachment(mailInfo.stream, mailInfo.filename));
                }
                smtpClient.Send(mailMsg);
            }
            catch (Exception ex)
            {
                return false;
            }
            return true;
        }

 public class EmailInfo
    {
        /// <summary>
        /// 收件人地址
        /// </summary>
        public string ToAddress { get; set; }
        /// <summary>
        /// 發件人地址
        /// </summary>
        public string FromAddress { get; set; }
        /// <summary>
        /// 顯示名稱
        /// </summary>
        public string DisplayName { get; set; }
        /// <summary>
        /// 郵件主題
        /// </summary>
        public string Subject { get; set; }
        /// <summary>
        /// SMTP服務器地址
        /// </summary>
        public string SmtpHost { get; set; }
        /// <summary>
        /// SMTP服務器端口
        /// </summary>
        public int SmtpPort { get; set; }
        /// <summary>
        /// SMTP服務器用戶名
        /// </summary>
        public string SmtpUserName { get; set; }
        /// <summary>
        /// SMTP服務器密碼
        /// </summary>
        public string SmtpPassword { get; set; }
        /// <summary>
        /// 郵件正文內容
        /// </summary>
        public string Body { get; set; }

        /// <summary>
        /// 附件 附件名字必填
        /// </summary>
        public Stream stream { get; set; }

        /// <summary>
        /// 附件名字：帶後綴的
        /// </summary>
        public string filename { get; set; }
    }

附件依賴於 NPOI



public static MemoryStream ExportCompanyOrder()
        {
            List<car_company_orderEntity> list = new List<car_company_orderEntity>();
            for (int i = 0; i < 100; i++)
            {
                list.Add(new car_company_orderEntity() { car_company_order_id = i, company_name = "測試" + i });
            }

            //創建Excel文件的對象
            NPOI.HSSF.UserModel.HSSFWorkbook book = new NPOI.HSSF.UserModel.HSSFWorkbook();
            //添加一個sheet
            NPOI.SS.UserModel.ISheet sheet1 = book.CreateSheet("Sheet1");
            //給sheet1添加第一行的頭部標題
            NPOI.SS.UserModel.IRow row1 = sheet1.CreateRow(0);
            row1.CreateCell(0).SetCellValue("ID");
            row1.CreateCell(1).SetCellValue("授權商名稱");

            //將數據逐步寫入sheet1各個行
            for (int i = 0; i < list.Count(); i++)
            {
                NPOI.SS.UserModel.IRow rowtemp = sheet1.CreateRow(i + 1);
                rowtemp.CreateCell(0).SetCellValue(list[i]?.car_company_order_id.ToString()); //ID
                rowtemp.CreateCell(1).SetCellValue(list[i]?.company_name); //授權商名稱
            }
            // 寫入到客戶端 
            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            book.Write(ms);
            ms.Seek(0, SeekOrigin.Begin);
            return ms;
        }

  var mailInfo = new EmailInfo();
            mailInfo.SmtpHost = "smtp.163.com";
            //mailInfo.SmtpPort = 25;
            mailInfo.SmtpUserName = "[email protected]";
            mailInfo.SmtpPassword = "cheng1993";
            mailInfo.DisplayName = "智領";
            mailInfo.FromAddress = "[email protected]";
            mailInfo.ToAddress = "[email protected]";
            mailInfo.Subject = "測試";
            mailInfo.Body = "測試的啊 ";
            mailInfo.filename = "111.xls";
            mailInfo.stream = ExportCompanyOrder();
            var aa = SendEmail(mailInfo);

demo 用來測試，以及發送。 註意：163後端需要在 開啟客戶端授權密碼

C# 163郵箱發送 附件以及郵件信息