首先NuGet 新增兩個包：MailKit和MimeKit

            MimeMessage message = new MimeMessage();

            message.From.Add(new MailboxAddress("發件人名稱", "[email protected]"));
            message.To.Add(new MailboxAddress("收件人名稱", "[email protected]"));

            message.Subject = "郵件標題"; 

            var builder = new BodyBuilder
            {
                HtmlBody = "郵件內容"
            };
            message.Body = builder.ToMessageBody();


            using (var client = new SmtpClient())
            {
                client.ServerCertificateValidationCallback = (s, c, h, e) => true;

                client.Connect("smtp.163.com", 465, true);

                client.Authenticate("[email protected]", "xxxx");

                try
                {
                    client.Send(message);//傳送郵件
                    client.Disconnect(true);
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex.Message);
                }
            }