A letter holder is an object used to store and organize letters. Postbag, ENVELOPE, mailbox, and post office box are all types of letter holders. A postbag is a bag used to carry mail from one place to another. An ENVELOPE is a paper container used to hold a single letter or document. A mailbox is a receptacle for receiving incoming mail. A post office box is a secure box located in a post office where people can receive mail without having to give out their home address.