It's fairly trivial to spoof the headers.
Yes, and analogously it's trivial to spoof the return address on postal mail by writing whatever you choose. It would still not be appropriate for someone to send out a bunch of mail pretending to represent someone that they do not, in attempt to get you to open and read it.
Addition: To those of you complaining about privacy - when you put something on the internet you shouldn't really have any expectation of privacy. Take this as a learning experience to use separate emails for work/school/play/etc.
This is a fair point, and while I'm not concerned with my email address being public, people submit their email because it is required -- for example to get forum notifications or password resets. If the forum is closed, they don't necessarily care whatever oatman currently thinks is cool several months down the road; certainly not enough to warrant sending an email posing as being sent from the official domain of the game.
Not to attack oatman specifically, since I'm sure he was just trying to share information about the game to some people who might be interested, but while I feel like this method was surely an effective and technically possible way of attracting new users, it was not ethically a good way of doing so.
As many have mentioned, don't click the link. it is a phishing email. Not a good thing!
The links in the email I received appear to be fine -- they just link to the official site. Unless you've received a different message, it's an unofficial email but not attempting to get any information from you.