How to send invites/activation links?

string activationCode = System.Guid.NewGuid().ToString();

        string bodyText = "<a href="$href$?ActivationCode=$ActivationCode$">Click Here</a>";

        string url = System.Web.HttpContext.Current.Request.Url.ToString();
        int i = url.LastIndexOf('/');
        url = url.Remove(i + 1);
        url += "Activate.aspx";

        bodyText = bodyText.Replace("$ActivationCode$", activationCode);
        bodyText = bodyText.Replace("$href$", url);
         
       SendEmail("Admin",
                                 "a@a.com",
                                 "hi",
                                 bodyText);

public static void SendEmail(string sender, string receiver, string subject, string body)
        {            
            string fromEmail = ConfigurationManager.AppSettings["ADMIN_EMAIL"].ToString();
            string password = ConfigurationManager.AppSettings["ADMIN_EMAIL_PASSWORD"].ToString();

            System.Net.NetworkCredential cred = new System.Net.NetworkCredential(fromEmail, password);

            string bodyText = Regex.Replace(body, @"<(.|\n)*?>", string.Empty);

            var plainView = AlternateView.CreateAlternateViewFromString(bodyText, null, "text/plain");
            var htmlView = AlternateView.CreateAlternateViewFromString(body, null, "text/html");

            var mail = new MailMessage
            {
                From = new System.Net.Mail.MailAddress(sender, "Farolito's Journey"),
                ReplyTo = new System.Net.Mail.MailAddress(sender, "Farolito's Journey"),
                BodyEncoding = Encoding.GetEncoding("utf-8"),                
                Subject = subject
            };

            mail.To.Add(new MailAddress(receiver));
            mail.AlternateViews.Add(plainView);
            mail.AlternateViews.Add(htmlView);
            mail.Priority = MailPriority.High;            

            var client = new SmtpClient
            {
                Host = ConfigurationManager.AppSettings["ADMIN_EMAIL_HOST"].ToString(),
                UseDefaultCredentials = false,                
                Credentials = cred
            };

            try
            {
                client.Send(mail);
            }
            catch (Exception)
            {
                
            }

            mail = null;
        }
    }

if (Request.QueryString["ActivationCode"] == null)
            Response.Redirect("Default.aspx");

        string activitionCode = Request.QueryString["ActivationCode"];