It might be quite disturbing to have the file open up every time you send mail.
This goes into the ThisOutlookSession module. It runs when Outlook is started.
Code:
Private Sub Application_Startup()
Dim oXLApp As Object
Dim oXLwb As Object
Dim oXLws As Object
On Error Resume Next
Set oXLApp = GetObject(, "Excel.Application")
If Err.Number <> 0 Then
Set oXLApp = CreateObject("Excel.Application")
End If
On Error GoTo 0
oXLApp.Visible = True
Set oXLwb = oXLApp.Workbooks.Open("h:\Test\test.xlsx")
Set oXLws = oXLwb.Sheets("Sheet2")
oXLws.Select
ExitRoutine:
Set oXLws = Nothing
Set oXLwb = Nothing
Set oXLApp = Nothing
End Sub