Perhaps no American principle of freedom is more zealously safeguarded than the privacy of your mail. Among US Postal Service employees, the term "Sanctity of the Seal" has long stood for mail privacy. It means that any free citizen of the United States can mail a letter with confidence that its seal will not be broken or its contents examined by anyone other than the addressee.
Privacy of first-class mail is an intrinsic right that has been enjoyed by US citizens for more than 200 years. Our founding fathers believed mail privacy was vital to the exercise of freedom; therefore, it must not be violated.
This principle is so strong that even when law enforcement agents believe that a first-class mailing may have contents related to a crime, they must seek a federal search warrant. Without a warrant signed by a federal judge, the mailing must remain undisturbed.
Mail privacy means that a parent who encloses a check in a birthday card and mails it to a daughter or son at college can expect it to arrive intact. It means that when businesses send bills and statements of account to customers, the information will remain confidential. It means that an affectionate card will not be examined by anyone other than its intended recipient.
On rare occasions, properly sealed letters may be damaged in handling by modern mail processing equipment. If this happens, the envelope is placed in a plastic wrapper and delivered. Only undeliverable first-class mail not bearing a return address may be opened (as a last resort) at one of the Postal Service's Mail Recovery Centers, where specially designated personnel will attempt to determine if the mail contains anything of monetary value and where it can be delivered.
The Postal Service welcomes the sacred trust of maintaining mail privacy and recognizes that customers must be able to rely on the Sanctity of the Seal in order to mail their letters with complete confidence. Employees of the Postal Service have earned this trust for more than 200 years.