Error in sending mail using SmtpClient

Dears Experts,
I tried to send email and attached a file to it from VB.NET code using the SmtpClient, but unfortunatily when I exexuted the code I got the following error. Could you please adivce

Module Module1

    Sub Main()
        pushMail("SDP-Daily", "SDP-Daily@mycom.com", "me@mycom.com", "SDP-Daily")
    End Sub
    Public Sub pushMail(ByVal senderName As String, ByVal from As String, ByVal toAddress As String, ByVal subject As String)
        Dim objMail As New System.Net.Mail.MailMessage
        Dim addrss As System.Net.Mail.MailAddress = New System.Net.Mail.MailAddress(from)
        objMail.From = addrss
        objMail.To.Add(toAddress)
        objMail.Subject = subject
        'objMail.IsBodyHtml = True
        objMail.Body = "test"
        Dim objAttch As Net.Mail.Attachment = New Net.Mail.Attachment("C:\SDP-07042008.xls")

        objMail.Attachments.Add(objAttch)
        Dim client As System.Net.Mail.SmtpClient = New System.Net.Mail.SmtpClient("RUH-003-MX-001") 'the Client used in our lan
        Try
            client.Send(objMail)
            objMail.To.Clear()
        Catch ex As Exception
        End Try
    End Sub
End Module
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