sending email problem

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ASP.NET

hi everybody,
i have serious problem in my web site.
i had use some sort of code to send email in my website.the problem is that code works properly in one page and don't work on the other page.
i'm completely confused.
the code behind and web.config :

MailMessage mail = new MailMessage();
 mail.From = new MailAddress("contacts@website.com");
 mail.To.Add(DropDownList1.SelectedValue);
mail.Subject = txtSubject.Text;
 mail.Body = "<div>something...</div>";
 mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient("mailserver.com");
smtp.Credentials = new NetworkCredential("contacts@website.com","password");
smtp.Send(mail);

W E B . C O N F I G

<pre lang="xml"><system.net>
    <mailSettings>
      <smtp>
        <network defaultCredentials="false"/>
      </smtp>
    </mailSettings>
  </system.net>

every solution will be appreciated
thanks