using System.Web.Mail;
MailMessage mail = new MailMessage();
mail.From = "maildemiempresa@xxxxxxxx.com";
mail.To = "xxx@msn.com";
mail.Subject = "prueba";
mail.Body = "este es un mail de prueba";
mail.BodyFormat = MailFormat.Text;
SmtpMail.SmtpServer = "dir ip del smtp";
SmtpMail.Send(mail);