Further to the mention last night of secret facilities operated by AT&T in the US where the NSA has access, this article <https://arstechnica.com/tech-policy/2018/06/meet-the-8-spooky-att-buildings-that-almost-certainly-also-serve-the-nsa/> reports on a list of eight likely places as detailed by The Intercept <https://theintercept.com/2018/06/25/att-internet-nsa-spy-hubs/>. While the NSA is not technically allowed to spy on wholly “domestic” communications, there seem to be no limits on any “incidental collection” as a result of targetting some foreign communication. That means they can grab the contents of entire mailboxes just for the sake of one message. Then keep the lot for whatever use they like.