A mailbag is a pouch which is designed to hold mail securely while protecting it from the elements. Mailbags can be used to ship and handle mail, in which case they are sometimes known as mail sacks, and they can also take the form of shoulder bags worn by mail delivery personnel. A number of legal mandates surround the use of mailbags, and in many cultures, there are rules of etiquette about mailbags similar to the rules which surround mail itself; for example, rummaging through a mailbag is illegal in many countries, in addition to being viewed as rude.
In the sense of a sack used to transport mail, a mailbag is usually large, to accommodate a high volume of mail, although small enough for a single mail thrower to move the bag. Typically the contents of a mailbag are sorted and the outside of the bag is tagged with the post office it should be directed to, ensuring that once a mailbag has been packed and sealed, it will not need to be reopened. When mailbags arrive at their end destination, postal employees break the seal, sort the mail by route, and then deliver it.
Businesses which get a high volume of mail, such as newspapers, may have a special arrangement with the post office which allows them to receive their mail in sacks, typically sorted by class. These businesses have their own mailrooms, with civilian staff who sort through the mail bags and route the mail to the appropriate department.
Because mail is federally protected in many nations, security of mailbags is of paramount importance. Typically they can only be handled by authorized postal personnel, and in many cases, mailbags are guarded when they are in open areas to ensure that they cannot be accessed by ordinary civilians. Many mailbags have barcodes which can be scanned at each step of their journey for tracking purposes, and if a mailbag contains registered mail or mail with special handling directions, extra special care will be taken.
In the sense of a bag carried by a mail carrier, a mailbag is much smaller, but still very sturdy, with a broad strap to distribute the weight of the mail. Mailbags can be slung over the shoulder for a route performed on foot, and some mail carriers carry mailbags in their vehicles so that they can park, load the mailbag with mail, and walk for part of their route, thereby getting exercise and saving fuel. Mail bags are also popular with non-postal personnel because of their sturdiness.