Work with Application.ItemSend Event and simple MsgBox Function
Also look at this answer Finding a workbook in one of multiple Excel instances
Example Code goes under ThisOutlookSession
Public WithEvents olApp As Outlook.Application
Private Sub Application_Startup()
Set olApp = Outlook.Application
End Sub
Private Sub olApp_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim xlApp As Excel.Application
Dim xlBook As Workbook
Dim Prompt As String
Prompt = "Open Excel File?"
If MsgBox(Prompt, vbYesNo + vbQuestion, _
"Sample") = vbNo Then
Cancel = True
Else
Set xlApp = New Excel.Application
Set xlBook = xlApp.Workbooks.Open( _
"C:\Temp\Temp.xlsm")
xlApp.Visible = True
End If
Set xlApp = Nothing
Set Book = Nothing
End Sub