The IRS runs a number of tax audits each year, and as such, has to obtain information on private citizens. If the information is in a physical format, the agency must obtain a warrant to access it. If it's stored online via email or other electronic information, there is no such protection.
In a Freedom of Information Act request, the ACLU obtained a number of IRS documents that explain the agency's rules in regards to obtaining digital information. Much like other law enforcement agencies, the IRS operates under the ECPA, a decades-old law that allows government agencies to obtain emails without a warrant if said email has been opened or is more than 180 days old.
So far, all of this is old news. What's the IRS doing that's so different from any other agency? In the official IRS search warrant handbook from 2009, the agency's guideline explicitly states that the Fourth Amendment doesn't apply to online communications. Here's the relevant portion of the handbook:
"...the Fourth Amendment does not protect communications held in electronic storage, such as email messages stored on a server, because internet users do not have a reasonable expectation of privacy in such communications.”
To make matters worse, the IRS Office of Chief Counsel reiterated this line of thinking a year later when they said that the Fourth Amendment does not "protect emails stored on server." The ACLU points to other documents that imply the IRS is obtaining emails left and right without a warrant all thanks to the ECPA's outdated definitions.
It's no surprise to see the IRS taking advantage of the Fourth Amendment loophole in the ECPA. The surprising part is just how frank the agency is about its data collecting methods. It's also depressing to see that the agency feels that American citizens "do not have a reasonable expectation of privacy" on the Internet.
Of course, all of that should have changed in 2010 with the United States v. Warshak, a Sixth Circuit Appeals Court ruling that found law enforcement had violated a man's Fourth Amendment rights when they obtained his emails without a search warrant. Unfortunately, the IRS feels that it would only need to consider obtaining a warrant when dealing with cases in the sixth circuit. It's still open season for warrantless email collection everywhere else.
It's a little distressing to find that the IRS holds Americans' Fourth Amendment protections in such low regard, but it's only foolish at this point to think any government agency actually cares about the Fourth Amendment in regards to online communications. We can only hope that Congress passes one of the many bills it's proposing this year to reform the ECPA.