By law , first-class mail is sealed against inspection, meaning that government officials may not open it without first getting a warrant from a judge. A citizen would be forgiven for imagining that this law ensures his or her mail is private, but that’s not quite true.
For one thing, other classes of mail do not have the same privacy protection as first-class mail. Additionally, thanks to GovernmentAttic.org , we now know the federal government has established three broad exceptions to first-class mail’s “sealed against inspection” protection.
A 2007 U.S. Department of Justice  letter lays out the circumstances under which mail may be opened without a warrant.
First, it has long been recognized that the Fourth Amendment permits the warrantless opening of mail under “exigent circumstances.” The Supreme Court has stated that “[t]he need to protect or preserve life or avoid serious injury is justification” for a warrantless search…. The Postal Service informs us that “exigent circumstance” searches typically are initiated when a postal inspector observes a suspicious package. Packages may be suspicious, for example, because they are vibrating, making noises, or leaking suspicious substances…. (See 39 CFR 233.11 .)
Second, the Fourth Amendment permits the warrantless searching of mail entering or leaving the United States…. Congress specifically has authorized the warrantless search of mail at the border, although some of those provisions place restrictions on the reading of correspondence. See, e.g., 19 U.S.C. § 1583 (a)(l) (permitting warrantless search of mail of domestic origin transmitted for export … and foreign mail transiting the United States”), (c)(1)-(2) (permitting search of first-class mail weighing more than 16 ounces if there is reasonable cause to believe that the mail contains specified contraband, merchandise, national defense or related information, or a weapon of mass destruction, but requiring a judicial warrant or consent to read any correspondence such mail contains)….
Third, provisions in the Foreign Intelligence Surveillance Act of 1978 (“FISA”), as amended, 50 U.S.C. § 1801 et seq., specifically authorize the Attorney General to conduct physical searches of mall without prior judicial authorization in certain circumstances. Section 304(e) of FISA, 50 U.S.C. § 1824 (e), provides that the Attorney General, under certain circumstances, may approve the execution of an emergency physical search of property, including property that “is in transit to or from an agent of a foreign power or a foreign power,” id. § 1824(a)(3)(B), so long as the Attorney General subsequently obtains an order from the Foreign Intelligence Surveillance Court authorizing the search….
The DOJ letter may be read in full here  (see pages 12-15.) It is worthy of close examination, as it defends a presidential signing statement  that is at odds with the plain language of a 2006 act of Congress .
The expectation that personal correspondence should remain private is centuries old. In the 1750s, for example, Postmaster Benjamin Franklin instituted a policy forbidding postmasters from reading individuals’ letters.
So it is dismaying that the Postal Service , the Inspection Service and the DOJ are not upfront with the public as to when they feel fit to open private mail. Indeed, the above DOJ letter came to light because GovernmentAttic.org  used the Freedom of Information Act  to shake it loose from DOJ. And this policy letter was written only because a pesky congressman demanded an answer from DOJ.
President Obama promised his administration would establish “an unprecedented level of openness”  and transparency in government. In this instance, that standard was not met.
- “By law”: http://www.law.cornell.edu/uscode/text/39/404
- “GovernmentAttic.org”: http://www.governmentattic.org/
- “2007 U.S. Department of Justice”: http://www.governmentattic.org/13docs/DOJviewsLetters_2005-2007.pdf
- “39 CFR 233.11”: http://www.ecfr.gov/cgi-bin/retrieveECFR?gp=1&SID=6b9726d27974b907c0f76b92f3a4fdfc&h=L&n=pt39.1.233&r=PART&ty=HTML#se39.1.233_111
- “19 U.S.C. § 1583”: http://www.law.cornell.edu/uscode/text/19/1583
- “50 U.S.C. § 1824”: http://www.law.cornell.edu/uscode/text/50/1824
- “full here”: http://www.governmentattic.org/13docs/DOJviewsLetters_2005-2007.pdf
- “presidential signing statement”: http://georgewbush-whitehouse.archives.gov/news/releases/2006/12/20061220-6.html
- “2006 act of Congress”: http://maloney.house.gov/sites/maloney.house.gov/files/documents/govreform/postal/20070122_CRSReportpostalaccountability.pdf
- “Postal Service”: https://about.usps.com/who-we-are/privacy-policy/faqs.htm
- “GovernmentAttic.org”: http://httt.www.governmentattic.org
- “Freedom of Information Act”: http://fas.org/sgp/crs/secrecy/R41933.pdf
- ““an unprecedented level of openness””: http://www.whitehouse.gov/the_press_office/TransparencyandOpenGovernment