The Google Webmaster Central Blog announced they will be resuming the emailing of penalty notifications to webmasters.
Google stopped sending these notifications when they learned that some people were using these notifications as a way to scare, spam and send viruses to webmasters. Soon after, Google launched a message center to notify webmasters who have registered with Google Webmaster Central about penalties that have been applied to their site, from within that system.
Not everyone is registered, of course. That is why Google has resumed the email portion of notifying webmasters of Google penalties.
How do you know if a Google penalty email is from Google? Google says that their emails will “not include attachments,” so any emails with attachments from Google on this matter are fake.
In the future, Google hopes to tell webmasters to login to Google Webmaster Central or register their sites with Google Webmaster Central to confirm and validate that their site has been penalized. But currently, if you register a new account with Google Webmaster Tools, old penalty notifications sent before you registered will not show up. Google hopes to change that to show past notifications, as well.