The email, with a subject heading of "Update about The Sims Social", was sent from "thesimssocialemail.com" which was registered by the Corporation Service Company (CSC), so it checks out as legit.
In it, the staff of the game thanks the fans for being "adventurous early adopters", promises us that they're working hard on improving the game, and that -- here's the most important bit --
This sounds like they're getting ready to let folks back in, but it's not going to be all of us at once. Aside from that, I guess they're being nice and just wanted to emphasize to players that the game's still in beta, which is why they refer to game as "The Sims Social Beta" in the opening and closing paragraphs. Because hey, if you look carefully, you'll also see that they've finally tacked the word "beta" onto the game's logo.
Part of of this beta testing stage is gradually adding more players in a controlled way. So if you are currently not able to play The Sims Social, this is why. Please bear with us while we go through this process and rest assured that you will not lose your progress in the game to date.