Natboxes blocking SMTP

That would work for about 4 hours, and then the zombies would start re-opening it with uPnP. Just as they would start "authenticating" the emails as "paired", based on your Outlook address book.

Well, OK, maybe not _yours_, but the vast majority of folks who host zombies would be Outlook Express users (with the preview pane enabled, yet), would only accept "paired email" if it was completely transparent to them, and if they use a NatBox at all, would have uPnP enabled.