A client of ours arrived at work to discover an email from the IRS in her inbox. Concerned about it's legitimacy she contacted us. Below is a copy of the email.
The first hint that this is fake, for those who haven't heard the IRS announcements that they will never email you, is the email was in her Junk mail. Typically important / legitimate emails don't get routed directly to the Junk mail box. Sure it happens occasionally, but it's certainly an easy first test of legitimacy.
As per their MO we see the scammers are hoping to work on people's fear instead of common sense, with the last line being the threat of "Failure to comply..."