Basically, masonry is referring to any material that is put together in units and bound in addition to what is called mortar. Mortar is short, the "glue" of those sorts of things. There are a several different materials which you can use in brick mailboxes, including granite, brick, marble, limestone, https://cirandas.net/gearweed31/blog/masonry-repair-restoring-the-strength-and-beauty-of-brick-and-stone-structures