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The process governed by the U.S. Postal Regulations (39 C.F.R. § 233.3) that allows the recording of all the information that appears on the outside cover of mail in any class, and also allows the recording of the contents of second-, third-, and fourth-class mail, international parcel post mail, and mail on which the appropriate postage has not been paid.
Mail covers may be granted by the chief postal inspector, or a delegate of the inspector's, and are allowed upon the request of a law enforcement agency. The law enforcement agency's purpose must be to protect national security, locate a fugitive, obtain evidence of the commission or attempted commission of a crime, or help identify property, proceeds or assets forfeitable under law.
To obtain a mail cover, the law enforcement agency must make a request in writing to the chief postal inspector, and must specify reasonable grounds demonstrating the necessity of the mail cover. The regulations do not define reasonable grounds, but in Vreeken v. Davis, 718 F.2d 343 (1983), the Tenth Circuit Court of Appeals held that a statement as to why the mail cover was necessary to an investigation, and that the subjects of the mail cover were under Grand Jury investigation, was sufficient. In Vreeken the court held that a letter stating that the plaintiffs were subjects of a grand jury investigation for tax Fraud, and that the mail cover was necessary to identify promoters, finders, and investors involved in the alleged scheme, was enough to meet the requirements of the mail cover regulations. The court stated that the regulations do not include a requirement that the request contain "the factual predicate upon which it concludes that the subject of the mail cover is involved in the commission of a crime."
The constitutionality of mail cover has been challenged primarily as a violation of the Fourth Amendment right against unreasonable searches and seizures. Although the U.S. Supreme Court has not addressed this issue directly, lower courts have held that such a violation does not exist. Mail cover has been compared to the use of a Pen Register, which is a mechanical device that records the numbers dialed on a telephone without monitoring the conversation. The Supreme Court, in Smith v. Maryland, 442 U.S. 735, 99 S. Ct. 2577, 61 L. Ed. 2d 220 (1979), held that pen registers do not violate an individual's Fourth Amendment right to privacy. The Court concluded that there is no reasonable expectation of privacy regarding the numbers dialed on a telephone because the user knows that the phone company receives those numbers. The court in Vreeken compared mail covers to pen registers in that the contents of mail are not examined, and that a person sending or receiving mail should know that the information first goes to the post office and that the outside of the mail must be examined by employees of the post office before it can be delivered.
Feld, Daniel E. 1982. "Validity, under Fourth Amendment, of 'Mail Cover.'" American Law Reports 57.