using System.Web.Mail;
...
MailMessage mail = new MailMessage();
mail.From = tbFrom.Text;
mail.To = tbTo.Text;
mail.Subject =  "Отчёт по ..." ;
mail.Attachments.Add(new MailAttachment(@ofdMain.FileName,MailEncoding.Base64));
SmtpMail.SmtpServer = tbSMTPServer.Text;
SmtpMail.Send(mail);