The mother of one of my colleagues received this email the other day:

There are a number of obvious things wrong with the email, including an exclamation point (not typically seen in IRS correspondence) and a copyright statement at the bottom, which would be really weird for an IRS notice.

The most obvious problem, though, is that the IRS never issues notices to taxpayers by e-mail. If you have received an "IRS notice" by e-mail, it's bogus. Do not open it, and especially do not click on any links in the e-mail.