Отправить письмо с вложением

            string smtpServer = SPAdministrationWebApplication.Local.
            OutboundMailServiceInstance.Server.Address;
            string smtpFrom = SPAdministrationWebApplication.
            Local.OutboundMailSenderAddress;

            MailAddress fromAdress = new MailAddress(from);
            MailAddress toAdress = new MailAddress(to);
            MailMessage mailMessage = new MailMessage(fromAdress, toAdress);

            mailMessage.Subject = title;

            mailMessage.Body = message;
            mailMessage.IsBodyHtml = true;
            WebClient oWebClient = new WebClient();
            oWebClient.Credentials = CredentialCache.DefaultNetworkCredentials;
            string attachment_url = "http://sitename/biblio/file.docx";
            byte[] data = oWebClient.DownloadData(attachment_url);
            MemoryStream oMemoryStream = new MemoryStream(data);
            string filename_attachment = "filename.docx";
            mailMessage.Attachments.Add(new System.Net.Mail.
                Attachment(oMemoryStream, filename_attachment, MediaTypeNames.Text.Plain));
            SmtpClient oSmtpClient = new SmtpClient(smtpServer);
            oSmtpClient.Send(mailMessage);

Если необходимо отправить письмо с пометкой важно, то мы просто добавляем:
mailMessage.Priority = MailPriority.High;