Internal memos from the Internal Revenue Service indicate that the agency would try to read personal emails without a warrant.
The American Civil Liberties Union obtained IRS memos asserting that people have no right to privacy online, including emails.
A 2009 policy statement from the IRS asserted that “emails and other transmissions generally lose their reasonable
expectation of privacy and thus their Fourth Amendment protection once
they have been sent from an individual’s computer.”
Even in 2010, after an appeals court ruled that emails fall under the Fourth Amendment protection from unreasonable search and seizure, IRS policies apparently indicated they should still be able to read personal emails.
Some email providers, including Google, Microsoft and Yahoo, have made public statements saying they would need a warrant to allow law enforcement to read emails.
See the story at CNET.