send automatic mail

[Roba roro]

hi ,
i'm trying to sned a mail in my web (ASP.NET) ,i want to be able to send mails to my clients who have hotmail yahoo or gmail accounts....from different accounts also ....

in web.config

<system.net>
    <mailSettings>
      <smtp from="" deliveryMethod="network">
        <network host="smtp.sigorta-online.com" port="25"  defaultCredentials="true" />
      </smtp>
    </mailSettings>
  </system.net>

my function is :

 Public Shared Function SendMail(ByVal strFrom As String, ByVal strTo As String, ByVal strSubject As String, ByVal strMsg As String) As Boolean
        Try
            ' Create the mail message
            Dim objMailMsg As MailMessage = New MailMessage(strFrom, strTo)
 
            objMailMsg.BodyEncoding = Encoding.UTF8
            objMailMsg.Subject = strSubject
            objMailMsg.Body = strMsg
            objMailMsg.Priority = MailPriority.High
            objMailMsg.IsBodyHtml = True
 
            'prepare to send mail via SMTP transport
            Dim objSMTPClient As SmtpClient = New SmtpClient()
            'objSMTPClient.EnableSsl = True
            objSMTPClient.Credentials = CredentialCache.DefaultNetworkCredentials
            objSMTPClient.UseDefaultCredentials = True
            objSMTPClient.DeliveryMethod = SmtpDeliveryMethod.Network
            objSMTPClient.Send(objMailMsg)
            Return True
        Catch ex As Exception
            Throw ex
        End Try
    End Function

it gives me this error message :
Mailbox unavailable. The server response was: <> No such user here
and when i put 'localhost" in my smtphost it gives me this error :
Failure sending mail.Unable to connect to the remote server
....any ideas ?