The law concerning marijuana and the mail is similar to that regarding the government's eavesdropping on telephone conversations. Like the contents of a telephone conversation, the contents of a letter or package are considered private by most reasonable people. The Supreme Court has held that United States first-class mail, whether a letter or package, is protected by the Fourth Amendment. Therefore, the government cannot open first-class mail on the mere hunch that it contains marijuana. Rather, the government must obtain a federal search warrant based on probable cause that marijuana will be found inside the letter or package. (A federal search warrant is required because state courts do not have jurisdiction over U.S. mail.)
Once a warrant has been obtained, officers can obtain the contents of your mail, either from the mail- stream or straight out of your mailbox. A government agent's failure to obtain a search warrant before opening first-class mail will result in the exclusion from court of any marijuana found during such an illegal mail search. Remember, however, only first-class mail is protected. This protection does not extend across boarders between US and Canda.