Mailbox unavailable. The server response was: <> No such user here

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

and this is the function
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("smtp.sigorta-online.com", 25)
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's giving this error
and now it's giving me this message :

Mailbox unavailable. The server response was: < > No such user here
what ever the mail adress i used it's giving me the same error

 

Dear Bruce i tried what you said and now its giving this error message :
Failure sending mail.Unable to connect to the remote server

Please i would appreciate your assisstance....what i want to do is to send an automatic mail from my ASP.net Website to the clients ..they may have yahoo, hotmail or gmail accounts ....and the account from where i'm sending may be static (but better if it can be variable ).......please do help