Mail collection service - letter boxes located on street corners for the deposit and collection of mail. Introduced in large U. S. cities in 1858, letters dropped in street boxes required an additional prepaid fee of 1-cent, but beginning in 1863 the collection service was performed free of charge.

Mailbag - the bag in which mail is transported by mailman, train, or some other means of conveyance

Mailbox - the container used to collect mail. Also known as 'collection boxes' (where people can deposit their mail for later pickup). Mailboxes are also used at households and businesses to receive mail.

Marine insurance stamp - a stamp associated with the Netherlands and Netherlands Indies prepaying a fee to carry mail in a shipboard safe designed to float free in case of sinking. Also a special postal tariff instituted by U.P.U. in 1920 following losses in WWI. Abolished in 1924.

Master die - the die used in the embossing process and refers to original engraving in recess

Maximum card - a picture postcard bearing a postage stamp and a postmark relevant to the picture on the card. The design on the stamp usually mirrors that on the postcard. The postage stamp is usually affixed to the picture-side to heighten the effect. This style of use was begun in Europe at the beginning of the twentieth century and remains popular.

Metal currency stamp - a stamp for use on foreign parcels which had to be prepaid at the 'gold' exchange rate in hard currency (gold or silver). Internal mail, on the other hand, used stamps purchased in depreciating paper currency. Stamps of Peru overprinted 'Plata' or silver are one example.

Metered mail - mail whose postage is paid by a machine that tracks the amount of postage applied by the machines

Migratory bird hunting stamp - a stamp produced annually for the Department of the Interior. Revenue from the sale of these stamps is used for waterfowl conservation. These are popularly called 'Duck Stamps'.

Military franchise stamp - a stamp allowing free postage of mail by military forces on active duty
Military stamp - a stamp used by military organizations and their personnel during both wartime and peace-keeping missions

Millesime - a numeral denoting the year of printing on many French and French colonial stamps. The millesime was located in the gutter next to the second row of stamps. Black numbers in the bottom right corner denote the year of printing after 1923.

Mochila - a Spanish term for knapsack. The mochilas used by Pony Express riders were made of leather and designed with four pockets, or cantinas, in which to carry mail.

Money Order - an order for the payment of money, usually issued by and payable at a bank or post office. Postal money orders originated in 1864. The maximum amount available at that time was 30-dollars.

Mourning stamp - a stamp issued on occasions of national bereavement such as the death of a president or king. The stamp might be a special issue printed in black with a black edge, or it might have the borders of existing stamps printed in black.

MPLSM - acronym for 'Multiple Position Letter Sorting Machines'. These machines operated through several work stations. Each worker sorted letters through keyboards attached to the machine. Workers read addresses and directed letters to the correct bin.