'open outlook and put this code into application session 

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim strMessage As String
Dim lngRes As Long
If Item.Subject = "" Then
    Cancel = True
    strMessage = "Please fill in the subject before sending."
    MsgBox strMessage, vbExclamation + vbSystemModal, "Missing Subject"
    Item.Display
End If
End Sub