Massachusetts has a unique history and legal system dating back to the original thirteen colonies. Service of process in Massachusetts is handled by constables, which are appointed by each county. You can become a constable without a specialized law enforcement background.
Each county maintains their own requirements and standards for appointment.
For an example, take a look at Boston’s constable application process.
While the process of becoming a constable can be tricky, the benefits include expanded powers to serve many forms of process not handled by process servers in other states.
For more information, see the Massachusetts Constables Association website.