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.^[A] 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.

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.