Mail Box Retrieval Question

Hi,
Who is allowed by contract or sanctity of the mail rules to go out on the street and pick up the mail boxes?  When hired in 1995 I was told only a B.E.M. or M.P.E. were allowed to touch them.  The Philadelphia policy was always to send 1 BEM and 1 MPE or 2 BEM’s.
Thanks,  Greg