The Internal Revenue Service (IRS) believes that they can read your e-mails without acquiring a warrant:
The Internal Revenue Service (IRS) has claimed that agents do not need warrants to read people’s emails, text messages and other private electronic communications, according to internal agency documents.
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In a 2009 handbook, the IRS said the Fourth Amendment does not protect emails because Internet users “do not have a reasonable expectation of privacy in such communications.” A 2010 presentation by the IRS Office of General Counsel reiterated the policy.
It’s fortunate that we live in a world where what the IRS thinks and what is actually true are two different things. While the Fourth Amendment doesn’t offer any protection from warrantless searches OpenPGP does. Enigmail is a plugin for Thunderbird that lets you easily encrypt your e-mail with Pretty Good Privacy (PGP). By encrypting your e-mail you can ensure only the intended recipient(s) can read it. Even if the IRS obtains a warrant to copy your e-mails from your service provider’s servers it won’t do them any good because they won’t be able to read those e-mails.
The IRS, or any other state agency, can only read your e-mail if you let them.