The email is legitimate.
Last year, Russian organized crime released a family of malware variously known as W32/DNSchanger, W32/Zlob, OSX.RSplug.A, OSX.DNSchanger, and so on.
What this malware did was point infected computers at hostile domain name servers run by organized crime. That way, they could control where you went on the Internet; for example, if you opened your browser and typed
www.paypal.com or a bank URL or whatever, they would redirect you to a fake site under their control and steal your login information. When you go to Google, they would intercept you, remove the Google ads, place their own ads, and then give you the Google page.
The FBI coordinated with law enforcement in several other countries and arrested all but one of the criminals responsible, who fled back to Russia. However, there are still tens of thousands of computers infected, and they are still using the hostile name servers.
So the FBI took over the name servers, because otherwise the computers that were still infected would be totally cut off from the Internet.
However, the FBI doesn't want to be in the name server business, so it's going to shut down those name servers soon. When they do, everyone who is infected and hasn't cleared out the infection will be unable to reach the Net.
The patch to fix the infection has been out for more than a year now, but still tens of thousands of computers are infected, which shows just how bad people (and businesses!) are about keeping secure. I know so many people who say things like "I don't understand why everyone freaks out about computer viruses, I've never been infected" who are probably infected sixteen ways from Sunday and simply don't know it.
There's an interesting postscript to this story. The one guy who escaped the law enforcement raid and fled back to Russia? He is believed to be the person responsible for the Mac Flashback malware.
