Mail sack

A mail sack or mailsack is a mail bag used to carry large quantities of mail.
Different handling and security requirements for different classes of mail is integral to the postal rate structure.
A mail sack is not a locked bag since they need little security. In contrast to a similar "mailbag" referred to as a mail pouch (for more sensitive mail such as personal letters and military mail) that employs a locking mechanism on the top of the bag. A "mail pouch" has special closely spaced eyelets and a strong strap to secure the top where access into the bag is closed off and locked, where a "mail sack" has none of these features.
During World War I it was typical of German soldiers to write postcards to their family to keep in touch to let them know where they were and what they were doing. The various ultimate destinations of the postcards were sorted into German "mail sacks" of that time period (1914–1918) by behind the scenes "post-office troops."
In the United Kingdom, the term "mail sack" is more expansive and generic, and typically involves larger bags that contain mail destined for one destination.
A Singapore judge held that mail sacks are considered to be part of the postal system and are protected by Chinese law; interference with them can be the subject of criminal prosecution.
United States Postal Service.
Second-class mail.
Second-class mail that would be carried in a "mail sack" is periodical publications issued at stated intervals and is issued a minimum of four times a year. This type of mail must have a date of issue and a consecutive numbering system. It also must have a real office where the publication comes from that is open during normal regular hours of business. The printed matter can not be stenciled, mimeographed or through a hectograph process. Second-class mail must be publications for distributing information of a public character ("e.g.", literature, sciences, industry information). The people that receive this second-class mail must be subscribers that are on some sort of list or in a customer database.
Third-class mail.
Third-class mail that would be carried in a "mail sack" is printed material weighing less than 16 ounces. Examples are circulars that are not of a personal nature (e.g. mass general public advertising, direct advertising mailing campaigns). Other third-class mail that would be carried in a "mail sack" is bulk mail that is presorted individually addressed letters that come in quantities of at least 50 pounds or mailings of over 200 pieces.
Fourth-class mail.
Fourth-class mail that would be carried in a "mail sack" is printed material weighing over 16 ounces. An example would be library books transferred through the interlibrary loan system.
M-bags, also called "Direct Sacks".
A special type of mail sack is one that is used to transport bulk mail, particularly printed matter, to a single recipient at a single foreign address. There is no minimum weight, and the maximum is a tare weight of 66 lbs. An M-bag is 4 feet tall and 36 inches in diameter. Every piece of mail in an "M-bag must be marked 'Postage Paid - M-bag.'" This type of shipping is especially prized by book stores and peace corps volunteers. It is an economical and secure way to move a lot of mail over a great distance on the surface (not air mail). Plans to discontinue this service originating in the USA in 2007 met with opposition. In August, 2007, subsidized rates were drastically curtailed. This gave rise to "complaints from everyone from scholars to record-album collectors - all of whom bemoan the loss of subsidized rates for research and educational (which includes entertainment) materials." See Private Mail Bag.
Foreign mail sack.
A mailbag used to transfer mail to a country other than the United States is defined as a "foreign mail sack". The normal design is that the top is closed with a drawstring like a leather strap. It is sealed using a lead seal (not a lock as in a mail pouch).
Australian post pickup service.
Australia has pre-paid mail sack service within the country. A person buys a pre-paid "satchel" (mail sack) to put mail into. The "satchels" (mail sacks) can be bought in bulk (sets of 10) for further discount. The sizes are 500 gram, 3 kg, and 5 kg. The "satchel" (mail sack) full of mail is then placed into an Australian red post box at certain street locations for pick-up. The "satchels" (mail sacks) can also be taken to the nearest post office for processing, especially if it is too big to fit through the opening of the street red post box. Certain paperwork must be filled out, especially if it is going by airmail. A proof of mailing is available with an additional fee paid at the post office (Australian retail outlet). There is an online tracking tool available for this pre-paid mail sack service.
New Zealand international mail.
New Zealand International Air Satchel is a discount service for the transport of large amounts of international mail from New Zealand. The mail is put into blue mail "satchel" bags (a type of mail sack provided by the New Zealand post office) and has free pick-up service available. The delivery of this international mail service from New Zealand is projected to be 3 to 10 working days. The discount service applies if a shipment of international mail is 100 New Zealand dollars or more. The requirements for this discount international mail service is that it must be over 50 international airmail letters or a set of small packages with a total mail fee cost of over 100 New Zealand dollars. The packages must be under 2 kg each. International airmail letters must weigh under 200 grams each, otherwise they are charged as a parcel package. If certain paperwork is filled out and pre-arrangements are made then a free courier pick-up service is available (1 per day).