The Collaborative International Dictionary
mailbag \mail"bag`\, mail bag \mail bag\n. A bag in which mailed matter is conveyed or shipped under public authority.
Syn: mail pouch.
2. A letter carrier's shoulder bag; as, in England they call a mailbag a postbag.
Syn: postbag.
Wikipedia
A mail bag or mailbag is a generic term for a type of bag used for collecting, carrying, categorizing, and classifying different types of postal material, depending on its priority, destination, and method of transport. It is oftentimes used by a post office system in transporting these different grades of mail. The mailbag is carried by some means of transporting like a mail carrier, animal (e.g., mule, horse), or a mobile post office. Letters and printed material delivered by mail in the seventeen-hundreds were carried by horse in a saddle bag. There are several different types of mailbags for different purposes (e.g., transporting mail to and from post offices, delivering mail to businesses and homes. These different styles of mailbags depend on its size and purpose. It can range from "a large bag used for transporting mail on a truck, plane, etc." to a simple "postbag" used by a mail carrier to deliver mail.
The idea of having mail bags on board ships traveling between Jamaica and Great Britain was established as early as 1780. The name of the ship carrying a letter was put on the corner of the letter so that it would be put into the proper mail bag for the destination intended.
A mailbag throughout the United States history has been called various names depending on its form and function at the time, some of which are now obsolete. Among these names are mail sack, mail satchel, mail pouch, catcher pouch, mochila, and portmanteau.
Private Mail Bags or so-called "Locked Bags" are a worldwide solution for specialized mail delivery to a single location. Like PO Box addresses, Private Mail Bag addresses omit the name of the building and street, and include only the number allocated to the user. Private Mail Bag addresses are often used in countries in Africa where there may be no street delivery service. In Europe and North America, where street delivery is more commonplace, large users may be allocated their own postal codes, and consequently need only use their physical address in correspondence; the postal code implies that the recipient receives mail by caller service. Private mail bags may be in lieu of a Post Office Box, but sometimes can go so far as to have an individualized corporate zip code.
Usage examples of "mail bag".
The letter was posted in the Milagro Post Office at the back of Rael's store, and the postmaster, Nick Rael, threw it into the mail bag which would be picked up by Teddy Martinez and taken to Chamisaville for sorting and canceling, a process that might take two days, and then Teddy Martinez would bring the bag of Milagro mail back up to Rael's' store, where it would sit for a day or so until Nick, or his kid, Jerry, got around to putting it out in the boxes.
The eighty-five cent dinner tasted like a discarded mail bag and was served to me by a waiter who looked as if he would slug me for a quarter, cut my throat for six bits, and bury me at sea in a barrel of concrete for a dollar and a half, plus sales tax.
On the morning of the sixth we passed through Blanc Sablon, the boundary line between Newfoundland and Canadian territory, and here I left the Newfoundland letters from my mail bag.
Jennifer slipped the tamperproof envelope into a bigger Priority Mail bag, and grabbed a ballpoint pen to fill out the adhesive waybill.
Just as he said it, the old man found July's letter under a mail bag.
Baccalieu, long delayed by weather, had finally reached Burgeo- Claire had walked to the post office and had staggered home under the weight of a full mail bag.