SMTP Sending Mails

my portal sending activation link to user to activate thier accounts

i notice that alot of user dosn't rescive activation mail and all those user are  using yahoo mail server

dose any one know something about this

 

my code to send e-mail :

SmtpMail.SmtpServer = System.Configuration.ConfigurationManager.AppSettings["SmtpServer"];
        MailMessage mail = new MailMessage();
        mail.BodyEncoding = System.Text.Encoding.GetEncoding("Windows-1256");
        mail.BodyFormat = MailFormat.Html;
        mail.To = email;
        mail.From = System.Configuration.ConfigurationManager.AppSettings["MailFrom"];
        mail.Subject = Subject;
   &nbs ...