No it isn't. The email passes through the server unencrypted (between Postfix and Dovecot), so it's still a potential tap point. In fact, running encryption to the server simply says to the NSA: Ask Linode for access to this disk.

I wonder how effective this advice would even be at achieving that goal. Consider, for instance, that what meager protections are afforded electronic communications (by the Stored Communications Act, etc.) only apply to providers that "serve the public". [1] Others, such as Jake Appelbaum, have argued that hosting with Gmail or other large providers offloads the legal burden onto their well-equipped legal teams to defend against subpoenas, FISA requests, NSL's and the like. [2]

The problem here is that it is very difficult to protect yourself when you don't understand your adversary's capabilities. We need transparent legislation and accountability in both government agencies and tech industry corporations. A lot of this advice boils down to an assumption that the NSA/DOJ/FBI will "play fair", and there is really no reason to expect them to do that.

[1]: https://ssd.eff.org/3rdparties/govt/stronger-protection [2]: http://youtu.be/HHoJ9pQ0cn8