Still unclear is whether taxpayers' other private communications, including on Facebook and Twitter, will receive the same treatment.
The ACLU has obtained internal IRS documents that say Americans enjoy "generally no privacy" in their e-mail messages, Facebook chats, and other electronic communications.
Democratic Sen. Mark Udall says the Justice Department should not allow FBI agents to peruse Americans' private communications without obtaining a search warrant from a judge.
An FBI investigation manual updated last year, obtained by the ACLU, says it's possible to warrantlessly obtain Americans' e-mail "without running afoul" of the Fourth Amendment.
A dozen senators, including Democrats and Republicans, want the IRS to pledge publicly not to snoop on Americans' Twitter and Facebook messages and other correspondence without a warrant.
Rebuffing the Justice Department, judges insist on warrants because e-mail records give police "the ability to peer deeply" into someone's activities.
Police investigations and "human life" would be jeopardized if search warrants were required for e-mail and location data, Justice Department official warns.
An appeals court decision, assuming it survives and is adopted broadly, marks a major turning point in the evolution of Fourth Amendment law in the Digital Age, consultant Larry Downes argues.
Dispute pitting Justice Department against business and civil liberties groups tests Obama's pledge "to strengthen both voluntary and legally required privacy protections."