/// <summary>
        /// 发送邮件(支持Html发送，支持添加一个附件)
        /// </summary>
        /// <param name="subject">邮件标题</param>
        /// <param name="email">收件人地址</param>
        /// <param name="content">邮件内容</param>
        /// <param name="filepath">文件的相对路径</param>
        public static void SendEmail(string subject, string email,string content,string filepath=null)
        {
            var message = new MimeMessage();
            //发信人
            message.From.Add(new MailboxAddress("发信人姓名", "发信人邮箱"));
            //收信人
            message.To.Add(new MailboxAddress("", email));
            //标题
            message.Subject = subject;
            //产生一个支持Html的TextPart
            var body = new TextPart(TextFormat.Html)
            {
                Text = content
            };
            //先产生一个
            var multipart = new Multipart("mixed");
            //添加正文内容
            multipart.Add(body);
            if (!string.IsNullOrWhiteSpace(filepath))
            {
                //生产一个绝对路径
                //filepath = "Upload//NewsPhoto//readme.txt";
                var absolutePath = Path.Combine(_hostingEnv.WebRootPath, string.Format(filepath));
                //附件
                var attachment = new MimePart()
                {
                    //读取文件(只能用绝对路径)
                    ContentObject = new ContentObject(File.OpenRead(absolutePath), ContentEncoding.Default),
                    ContentDisposition = new ContentDisposition(ContentDisposition.Attachment),
                    ContentTransferEncoding = ContentEncoding.Base64,
                    //文件名字
                    FileName = Path.GetFileName(absolutePath)
                };
                //添加附件
                multipart.Add(attachment);
            }    
            //正文内容
            message.Body = multipart;
            using (var client = new SmtpClient())
            {
                //连接到Smtp服务器
                client.Connect("smtp服务器地址", 端口, false);
                //登陆
                client.Authenticate("账号", "密码");
                //发送
                client.Send(message);
                //断开
                client.Disconnect(true);
            }
        }