If mail is searched or seized by law enforcement in Massachusetts, state courts apply federal law to determine whether the police conduct was lawful. See Commonwealth v. Pinto, 45 Mass. App. Ct. 790, 792 (1998).
Under federal law
Letters and other sealed packages are in the general class of effects in which the public at large has a legitimate expectation of privacy; warrantless searches of such effects are presumptively unreasonable.
United States v. Jacobsen, 466 U.S. 109, 114 (1984).
Based on this standard, police must have “reasonable suspicion” that the searched or seized mail is connected to a crime.
Again citing federal law, reasonable suspicion exits when police have “a particularized and objective basis for suspecting legal wrongdoing” United States v. Arvizu, 534 U.S. 266, 273 (2002).
There must also be a “legitimate expectation of privacy.” According to federal case law,
To successfully challenge a search on Fourth Amendment grounds, the defendant must show that he had a subjective expectation of privacy in the place searched that is accepted by society as objectively reasonable…When evaluating whether a person has a reasonable expectation of privacy, courts examine a variety of factors, such as ownership . . ., possession, access or control, ability to control.
United States v. FNU LNU, 544 F.3d 361, 365 (1st Cir. 2008)
Additionally, citizens may have an expectation of privacy in mail that is sent to them through a third party (i.e., a “bailee”). See United States v. Bates, 100 F. Supp. 3d 77, 83-84 (D. Mass. 2015).