Dim message As New MailMessage()
        With message
...
          .Attachments.Add(New Attachment(attach)) 'attach-путь к отправляемому файлу
        End With
        Dim client As New SmtpClient()
        With client
          ...
          If IsNothing(m_CallbackFunction) Then
             .Send(message)
            'в этом варианте точно могу здесь удалить attach
            Return True
          Else
            AddHandler client.SendCompleted, m_CallbackFunction
            .SendAsync(message, m_emailToken)
            'А вот здесь могу??? удалить attach???
            Return True
          End If
        End With

   Private Sub TimerMail_Tick(sender As Object, e As EventArgs) Handles TimerMail.Tick
     TimerMail.Enabled = False
     Do
        If (есть чего отправлять)
           отправляем кодом выше
        Else
           Exit Do
        End If
     Loop
     TimerMail.Enabled = True
  End Sub

Try : IO.Directory.Delete(IO.Path.Combine(IO.Path.GetTempPath, RandomFolder), True) : Catch : End Try

Try : IO.File.Copy(origfile, IO.Path.Combine(IO.Path.GetTempPath, origfileShortName, "FILE.pdf"), True) : Catch : End Try
attachfile = IO.Path.Combine(IO.Path.GetTempPath, origfileShortName, "FILE.pdf")

message.Attachments.Add(New Attachment(attachfile))

AddHandler client.SendCompleted, m_CallbackFunction
.SendAsync(origfileShortName, origfileShortName)

Private Sub SendCompletedCallback(ByVal sender As Object, ByVal e As AsyncCompletedEventArgs)
    Dim token As String = CStr(e.UserState)
...
    If IO.Directory.Exists(IO.Path.Combine(IO.Path.GetTempPath, token)) Then
      Try
        IO.Directory.Delete(IO.Path.Combine(IO.Path.GetTempPath, token), True)
      Catch ex As Exception
        Debug.Print(ex.Message) 'Процесс не может получить доступ к файлу "FILE.pdf", так как этот файл используется другим процессом.
      End Try
    End If
End Sub

14.04.2015 7:31:59	Sending mail asynchronously
		Token=140420150731146843887
14.04.2015 7:31:59	Sending mail asynchronously...waiting for the result
-------------------------
-------------------------
14.04.2015 7:32:00	Sending mail asynchronously...Failed. Error : Сбой при отправке сообщения электронной почты.
		Token=140420150731146843887
-------------------------

  <StructLayout(LayoutKind.Sequential)>
  Public Structure MyEmailToken
    Dim msg As System.Net.Mail.MailMessage
    Dim token As String
  End Structure

            AddHandler client.SendCompleted, m_CallbackFunction
            Dim MyToken As MyEmailToken
            MyToken.msg = message
            MyToken.token = m_emailToken 'здесь я в данной задаче передаю origfileShortName из %temp%\origfileShortName\attach.ext
            .SendAsync(message, MyToken)

  Private Sub SendCompletedCallback(ByVal sender As Object, ByVal e As AsyncCompletedEventArgs)
    ' Get the unique identifier for this asynchronous operation.
    Dim MyToken As MyEmailToken = CType(e.UserState, MyEmailToken)
    MyToken.msg.Dispose() 'удаляем message чтобы удалить папку с attach
...
    'удаляем временную папку с файлом 
    If IO.Directory.Exists(IO.Path.Combine(IO.Path.GetTempPath, MyToken.token)) Then
      Try : IO.Directory.Delete(IO.Path.Combine(IO.Path.GetTempPath, MyToken.token), True) : Catch : End Try
    End If
  End Sub

       Dim o_Mess As Object = CreateObject("CDO.Message")

        .AddAttachment(attach)
        .Send()
        'ну и можно удалять (здесь .Send ждет)

        'удаляем временную папку с attach
        DeleteFile attach
        Do 'цикл
          DoEvents
          If Len(Dir(GetEnvarVar("temp") & "\" & origfileShortName, vbDirectory)) > 0 Then
            rez = RemoveDirectory(GetEnvarVar("temp") & "\" & origfileShortName)
            If (rez = 1) Then 'если удалили выходим
              Exit Do
            End If
          Else 'если не сущ.выходим
            Exit Do
          End If
        Loop

        'удаляем временную папку с attach
        DeleteFile attach 'СНАЧАЛА ФАЙЛ
        Do 'цикл
          DoEvents
          If Len(Dir(GetEnvarVar("temp") & "\" & origfileShortName, vbDirectory)) > 0 Then
            rez = RemoveDirectory(GetEnvarVar("temp") & "\" & origfileShortName)
            If (rez = 1) Then 'если удалили выходим 'ДОЛБИМ ДО ПОБЕДНОГО
              Exit Do
            End If
          Else 'если не сущ.выходим
            Exit Do
          End If
        Loop

IO.Directory.Delete (включая содержимое)

Set o_Mess =Nothing

            IO.File.Delete(attach)
            Do 'цикл
              Application.DoEvents()
              If IO.Directory.Exists(IO.Path.Combine(IO.Path.GetTempPath, origfileShortName)) Then
                If RemoveDirectory(IO.Path.Combine(IO.Path.GetTempPath, origfileShortName)) Then
                  Exit Do
                Else
                  Debug.Print("Delete error")
                End If
                'Try
                '  IO.Directory.Delete(IO.Path.Combine(IO.Path.GetTempPath, origfileShortName), False)
                '  Exit Do
                'Catch
                'End Try
              Else 'если не сущ.выходим
                Exit Do
              End If
            Loop

        Dim o_Mess As Object = CreateObject("CDO.Message")

        With o_Mess
...
          .AddAttachment(attach)
...
          .Send()
          .Attachments.DeleteAll() 'принудительно отпускаем аттачменты с миром 
        End With

            If IO.Directory.Exists(IO.Path.Combine(IO.Path.GetTempPath, origfileShortName)) Then
              Try : IO.Directory.Delete(IO.Path.Combine(IO.Path.GetTempPath, origfileShortName), True) : Catch : End Try
            End If

Property Attachments As IBodyParts
    read-only
    Member of CDO.Message
    The object's Attachments collection.

Sub DeleteAll()
    Member of CDO.IBodyParts
    Deletes all BodyPart objects in the collection.

  Public Class MyEmailToken
    Public token As String
    Public msg As System.Net.Mail.MailMessage
    Public param As Object
  End Class

            AddHandler client.SendCompleted, m_CallbackFunction
            Dim MyToken As New MyEmailToken
            MyToken.token = m_emailToken
            MyToken.msg = message
            MyToken.param = m_param 'в m_param в нашем случае передаю полный путь %temp%\subfolder
            .SendAsync(message, MyToken)
 

  Private Sub SendCompletedCallback(ByVal sender As Object, ByVal e As AsyncCompletedEventArgs)
    ' Get the unique identifier for this asynchronous operation.
    Dim MyToken As MyEmailToken = CType(e.UserState, MyEmailToken)
    MyToken.msg.Dispose() 'удаляем message чтобы удалить папку с attach
    AddLogCDO("-------------------------")
    If e.Error IsNot Nothing Then
      Dim str_err As String = "Error : " & e.Error.Message
      AddLogCDO(Now.ToString & Chr(9) & "Sending mail asynchronously...Failed. " & str_err)
      AddLogCDO(Chr(9) & Chr(9) & "Token=" & MyToken.token)
      AddLogCDO("-------------------------")
    Else
      AddLogCDO(Now.ToString & Chr(9) & "Sending mail asynchronously...Done")
      AddLogCDO(Chr(9) & Chr(9) & "Token=" & MyToken.token)
      AddLogCDO("-------------------------")
    End If
    'удаляем временную папку
    If IO.Directory.Exists(CType(MyToken.param, String)) Then
      Try : IO.Directory.Delete(CType(MyToken.param, String), True) : Catch : End Try
    End If
  End Sub