"post, letters," c. 1200, "a traveling bag, sack for keeping small articles of personal property," a sense now obsolete, from Old French male "wallet, bag, bundle," from Frankish *malha or some other Germanic source, from Proto-Germanic *malho- (source also of Old High German malaha "wallet, bag," Middle Dutch male "bag"), from PIE *molko- "skin, bag."
The sense was extended to "bag full of letters" (1650s; perhaps via phrases such as a mail of letters, 1654) and "person or vehicle that carries postal matter" (1650s). From thence, to "letters and parcels" generally (1680s) and "the system of transmission by public post" (1690s).
As a newspaper name, by 1789. In 19c. England, mail was letters going abroad, while home dispatches were post. Sense of "a personal batch of letters" is from 1844, originally American English. Mail slot "narrow opening in an exterior door of a building to receive mail delivery" is by 1893, American English. OED defines it as a "letter-slit."
"metal ring armor," c. 1300, from Old French maille "link of mail, mesh of net," from Latin macula "mesh in a net," originally "spot, blemish," on notion that the gaps in a net or mesh looked like spots. Its use dates from late Roman times. The favorite armor in Europe 12c.-13c., it was effective, but heavy and costly.
"send by post," 1828, American English, from mail (n.1). "The usual word in the U.K. is still post" [OED]. Related: Mailed; mailing; mailable. Mailing list "register of addresses" is attested from 1876.
The origin, foundation and principle of mail order trading is universally recognized as wrong. It was conceived in iniquity and brought forth in despair as the world's greatest destructive medium. Mail Order Trading was born in the brain of knaves and thieves who fired their building for insurance profits, then sold the salvaged and damaged stock to the unsuspecting sons of man in distant territory. [Thomas J. Sullivan, "Merchants and Manufacturers on Trial," Chicago, 1914]
Even aerial navigation in 1999 was found too slow to convey and deliver the mails. The pneumatic tube system was even swifter, and with such facilities at hand it is not surprising that people in San Francisco received four daily editions of the Manhattan journals, although the distance between Sandy Hook and the Golden Gate is a matter of 3,600 miles. ["Looking Forward," Arthur Bird, 1899]
Associated Press style guide collapsed it to email 2011.
"having mail armor," late 14c., from mail (n.2). Of animals having protective skin or scales, by 1680s.