- post-office box, POB, call box, letter box - a numbered compartment in a post office where mail is put to be called for
--1 is a kind of compartment
- postbox, mailbox, letter box - public box for deposit of mail
--2 is a kind of maildrop
--2 has particulars: pillar box - mailbox, letter box - a private box for delivery of mail
--3 is a kind of box