จะทำ Window Form (VB.Net) ส่งเมล์ผ่านแต่ไม่ได้คับ ขอคำแนะนำหน่อยคับ Error : Failure sending mail.

    Public Sub SendMail(ByVal Filename As String, ByVal dept As String, ByVal remark As String)
        iConn.OpenOra(connection)/iConn.OpenSql(connection)
        Dim Smtp As New SmtpClient(Mail Server)
        Dim attFile As String = "D:\Export\" & Filename
        Dim mail As New MailMessage()
        If dept = "" Then
            cond = "where i_remark ='" & remark & "' "
        ElseIf remark = "" Then
            cond = "where i_dept ='" & dept & "' "
        End If
        'set the addresses
        Dim strsql As String = "Select I_mail_acc from T_mail_ms " & cond
        Dim da As New SqlDataAdapter(strsql, iConn.iConnSql)
        Dim dt As New DataTable()

        da.Fill(dt)

        If dt.Rows.Count <> 0 Then
            For i As Integer = 0 To dt.Rows.Count - 1
                Dim tomail As String = dt.Rows(i)("I_mail_acc").ToString()
                mail.To.Add(tomail)
            Next
        End If

        Smtp.Credentials = New System.Net.NetworkCredential("UserName", "PASSWORD")
        'set the content
        mail.From = New MailAddress("Email")
        mail.Attachments.Add(New Attachment("" & attFile & ""))
        mail.Subject = "This is an email"
        mail.Body = "Test Email "
        Try
            Smtp.Send(mail)
            MsgBox("Send Mail Complete!")
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Sub

Smtp.Send(mail)