A rack has a frame and a plurality of shelves. In one embodiment, a method includes creating a plurality of messages for transmission to one or more recipients, the plurality of messages including one or more real messages intended for one or more of the recipients and one or more bogus messages intended for none of the recipients. The method comprises transporting the microparticle to a selected location, irradiating a fluorescent substance tagged to the microparticle, and measuring the fluorescent light emitted from the fluorescent substance at the selected location.