attachment_resource

var message = new MimeMessage();
            message.To.Add(new MailboxAddress("xxxx@126.com", "xxxx@126.com"));//收件人
            message.From.Add(new MailboxAddress("xxxx", "xxxx@163.com"));//发件人
            message.Subject = "邮件测试主题2";

            var builder = new BodyBuilder();
            var image = builder.LinkedResources.Add(path);
            image.ContentId = MimeUtils.GenerateMessageId();

            builder.TextBody = string.Format(@"[cid:{0}] Hey AnLong", image.ContentId);
            builder.HtmlBody = string.Format(@"<p>Hey anlong</p><p>Will you be my +1</p><center><img src=""cid:{0}""></center>", image.ContentId);

            // We may also want to attach a calendar event for Monica's party...
            builder.Attachments.Add(@"C:\Users\Joey\Documents\party.ics");

            message.Body = builder.ToMessageBody();



            using (var client = new MailKit.Net.Smtp.SmtpClient())
            {
                client.Connect("smtp.163.com", 465, true);

                // Note: since we don't have an OAuth2 token, disable
                // the XOAUTH2 authentication mechanism.
                client.AuthenticationMechanisms.Remove("XOAUTH2");

                // Note: only needed if the SMTP server requires authentication
                client.Authenticate("xxxx@163.com", "123456");

                client.Send(message);
                client.Disconnect(true);
            }