Failure sending mail

hi
i'm trying tp send a mail in my ASP.net application i added the configuration of th SMTP to the Web.config:
<system.net>
    <mailSettings>
      <smtp from="" deliveryMethod="network">
        <network host="smtp.nameofsite.com" port="25"  defaultCredentials="true" />
      </smtp>
    </mailSettings>
  </system.net>
but its giving me this error :
System.Net.Mail.SmtpException was caught
  Message="Failure sending mail."
  Source="System"
  StackTrace:
       at System.Net.Mail.SmtpClient.Send(MailMessage message)
       at BRKTMEMO.SendMail(String strFrom, String strTo, String strS ...