Mail cover is a law enforcement investigative technique.

It is defined as follows by the Internal Revenue Manual:

Mail cover is the process by which a nonconsensual record is made of any data appearing on the outside cover of sealed or unsealed mail; or by which a record is made of the contents of any unsealed mail, as allowed by law, to obtain information to protect national security; locate a fugitive; obtain evidence of the commission or attempted commission of a crime; obtain evidence of a violation or attempted violation of a postal statute; or assist in the identification of property, proceeds, or assets forfeitable under law. http://www.irs.gov/irm/part9/ch04s13.html#d0e25473

In the United States, the United States Postal Service (USPS) regulations constitute the sole authority and procedure for initiating, processing, placing, and using mail covers, and are contained in Title 39, Code of Federal Regulations, Section 233.3 (39 CFR §233.3); and Section 213 of the USPS Administrative Support Manual.

Mail covers may not remain in effect for more than 120 days.http://edocket.access.gpo.gov/cfr_2002/julqtr/pdf/39cfr233.4.pdf

As mail cover does not involve the reading of the mail but only information on the outside of the envelope

Envelope

An envelope is a common packaging item, usually made of thin flat material. It is designed to contain a flat object, such as a letter or card....

 or package

Package

Package can refer to:* Packaging and labeling* Package testing* Mail item larger than a letter* Chip package or chip carrier, in electronics, the material added around a component or integrated circuit to allow it to be handled without damage and incorporated into a circuit* Automotive package, in...

 that could be read by anyone seeing the item anyway, it is not considered a violation of the Fourth Amendment

Fourth Amendment to the United States Constitution

The Fourth Amendment to the United States Constitution is the part of the Bill of Rights which guards against unreasonable searches and seizures, along with requiring any warrant to be judicially sanctioned and supported by probable cause...

. However, there has been criticism of the practice by some,http://everything2.com/index.pl?node_id=1281744 particularly due to the delay in mail the process might cause, though regulations prohibit mail cover from delaying mail.http://www.totse.com/en/conspiracy/institutional_analysis/postal.html