Essentially what Apple is going for with iOS. I honestly doubt they will allow them to do that for long. There's going to be a rematch eventually.
There are precedents for that kind of thing here and there already, in Bittorrent, Bitcoin, Ethereum, Freenet, TOR... and as [1] said, SMTP.
At some point that mail has to go into storage somewhere so it can be accessed by a user/client via IMAP, and that's where the subpoena/national security letter/warrant/FISA court request will go. To whatever location the disk storage resides at.