armor maker

letters; correspondence

correspondence university (abbr. for 函授大學|函授大学[han2 shou4 da4 xue2])

functor (math.)

to teach by correspondence

invitation letter; CL:封[feng1]

letter; piece of correspondence (incl. email)

incoming letter; letter from afar; same as 來信|来信[lai2 xin4]

an informal letter sent by an organization

official letter