MailMessage message = new MailMessage();
message.From = new MailAddress("from@server.com", "From Name");
message.To.Add(new MailAddress("to@server.com", "To Name"));
message.Subject = "Subject";
message.Body = "This is <b>HTML</b> text";
message.IsBodyHtml = true;

SmtpClient emailClient = new SmtpClient("mail.server.com");
emailClient.Send(message);