That doesn't do much to cloak the sender and receiver's ID or other metadata.
Well if that concerns you, you could set up your own private mail server on your own domain or even at just an ip address. If someone wanted to send you an anonymous (encrypted) message they could use a public internet hotspot and connect to your email server directly. Your traffic can still be sniffed and/or archived, but if it is encrypted there is no way to tie it to anyone.
I'm dubious about this excuse as to why Groklaw is closing. It certainly is their right to close, but this sows unnecessary fear, uncertainty and doubt. There is some speculation on Slashdot that Groklaw received a National Security letter of their own, but that doesn't seem likely. They aren't offering secure communications like Lavabit or Hushmail, so there isn't a reason for tap on their comms. It seems contrived.
Actually one would want to use an IP address that belongs to someone else, preferably on a cleanly compromised system, if one's goal was to prevent identification from the To: path, in the same way the sender would want to use a public access point with no way to ID the user.
Obviously other IP and user cloaking schemes could be used as well, but all of that pretty much eliminates the idea of a blog or newsroom with a "tip line".
All excessive surveillance does is encourage the development of alternative communication paths which are harder to gather intelligence from, which ultimately reduces the ability to gather necessary intelligence.