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