Problème d'envoi de mail avec VB.net

Bonjour,
voila j'essaie avec mon programme d'envoyer un mail à plusieurs contacts (placé dans un fichier excel) le probleme c'est que je ne reçois rien
Merci

Imports System.Net.Mail
Imports System.Web
Imports System.Data.OleDb
Imports System.Text
Imports System.Net.Mime
Public Class Form1

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim excel_conn As New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\contact.xls;Extended Properties='Excel 8.0;HDR=YES;'")
Dim r As OleDbCommand = excel_conn.CreateCommand
r.CommandText = "select * from [mail$]"
excel_conn.Open()

Dim mail As New MailMessage
Dim smtp As New SmtpClient("smtp.gmail.com")

smtp.Credentials = New System.Net.NetworkCredential("toto@gmail.com", "motdepass")
smtp.Port = "587"


Dim res As OleDbDataReader
res = r.ExecuteReader
While (res.Read)

smtp.EnableSsl = True
mail.To.Clear()
mail.To.Add(res.GetValue(0))
'mail.To.Add("titi@hotmail.com")
mail.From = New MailAddress("toto@gmail.com")
mail.Subject = "test"
mail.Body = "Message test"

Dim item As New System.Net.Mail.Attachment("C:\test.xls")
mail.Attachments.Add(item)

Try
smtp.Send(mail)
MsgBox("msg envoyé")
Catch es As Exception
MsgBox("Erreur dans l'envoi")
End Try
End While
excel_conn.Close()
End Sub
End Class