What is Mail?

Legal Definition
The mail or post is a system for physically transporting documents and other small packages; or, the postcards, letters, and parcels themselves. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century national postal systems have generally been established as government monopolies with a fee on the article prepaid. Proof of payment is often in the form of adhesive postage stamps, but postage meters are also used for bulk mailing. Modern private postal systems are typically distinguished from national postal agencies by the names "courier" or "delivery service".

Postal authorities often have functions other than transporting letters. In some countries, a postal, telegraph and telephone (PTT) service oversees the postal system, in addition to telephone and telegraph systems. Some countries' postal systems allow for savings accounts and handle applications for passports.

The Universal Postal Union (UPU), established in 1874, includes 192 member countries and sets the rules for international mail exchanges.
-- Wikipedia
Legal Definition
This word, derived from the French malle, a trunk, signifies the bag, valise, or other contrivance used in conveying through the post office, letters, packets, newspapers, pamphlets, and the like, from place to place, under the authority of the United States. The things thus carried are also called the mail.

2. The laws of the United States have provided for the punishment of robberies or wilful injuries to the mail; the act of March 3, 1825, 3 Story's Laws U. S. 1985, provides-

§22. That if any person shall rob any carrier of the mail of the United States, or other person entrusted, therewith, of such mail, or of part thereof, such offender or offenders shall, on conviction, be imprisoned not less than five years, nor exceeding ten years; and, if convicted a second time of a like offence, he or they shall suffer death; or if, in effecting such robbery of the mail, the first time, the offender shall wound the person having the custody thereof, or put his life in jeopardy, by the use of dangerous weapons, such offender or offenders shall suffer death. And if any person shall at- tempt to rob the mail of the United States, by assaulting the person having custody thereof, shooting at him, or his horse or mule, or, threatening him with dangerous weapons, and the robbery is not effected, every such offender, on conviction thereof, shall be punished by imprisonment, not less than two years, nor exceeding ten years. And, if any person shall steal the mail, or shall steal or take from, or out of, any mail, or from, or out of, any post office, any letter or packet; or, if any person shall take the mail, or any letter or packet therefrom, or from any post office, whether with or without the consent of the person having custody thereof, and shall open, embezzle, or destroy any such; mail, letter, or packet, the same containing any articles of value, or evidence of any debt, due, demand, right, or claim, or any release, receipt, acquittance, or discharge, or any other articles, paper, or thing, mentioned and described in the twenty-first section of this act; or, if any person shall, by fraud or deception, obtain from any person having custody thereof, any mail, letter, or packet, containing any article of value, or evidence thereof, or either of the writings referred to, or next above mentioned, such offender, or offenders, on conviction thereof, shall be imprisoned not less than two, nor exceeding ten years. And if any person shall take any letter, or packet, not containing any article of value, or. evidence thereof, out of a post office, or shall open any letter or packet, which shall have been in a post office, or in custody of a mail carrier, before it shall have been de-livered to the person to whom it is directed, with a design to obstruct the correspondence, to pry into another's business or secrets; or shall secrete, embezzle, or destroy, any such mall, letter, or packet, such offender, upon conviction, shall pay, for every such offence, a sum not exceeding five hundred dollars, and be imprisoned not exceeding twelve months.

3. - §23. That, if any person shall rip, cut, tear, burn, or otherwise injure, any valise, portmanteau, or other bag used, or designed to be used, by any person acting under the authority of the postmaster general, or any person in whom his powers are vested in a conveyance of any mail, letter packet, or newspaper, or pamphlet, or shall draw or break any staple, or loosen any part of any lock, chain, or strap, attached to, or belonging to any such valise, portmanteau, or bag, with an intent to rob, or steal any mail, letter, packet, newspaper, or pamphlet, or to render either of the same insecure, every such offender, upon conviction, shall, for every such offence, pay a sum, not less than one hundred dollars, nor exceeding five hundred-dollars, or be imprisoned not leas than one year, nor exceeding three years, at the discretion of the court before whom such conviction is had.

4. - §24. That every person who, from and after the passage of this act, shall procure, and advise, or assist, in the doing or perpetration of any of the acts or crimes by this act forbidden, shall be subject to the same pen-alties and punishments as the persons are subject to, who shall actually do or perpetrate any of the said acts or crimes, according, to the provision of this act.

5.- §25. That every person who shall be imprisoned by a judgment of court, under and by virtue of the twenty-first, twenty-second, twenty-third, or, twenty-fourth sections of this act, shall be kept at hard labor during the period of such imprisonment.
-- Bouviers Law Dictionary
Legal Definition
As applied to the post-office, this term mcans the carriage of letters, whether applied to the bag into which they are put, the coach or vehicle by means of which they are transported or any other means employed for their carriage and delivery by public authority. Wynen v. Schappert, 6 Daly (N. Y.) 560. It may also denote the letters or other matter so carried. The term "mail," as used in Rev. St U. S. § 5469 (In S. Comp. St. 1901, p. 3692) relative to robbing the mails, may mean either the whole body of matter transported by the postal agents, or any letter or package forming a component part of it U. S. v. Inabnet (D. C.) 41 Fed. 130. Mail also denotes armor, as in the phrase a "coat of mail." In Scotch law. Rent; a rent or tribute. A tenant who pays a rent is called a "mall-payer," "mailer," or "mail-man." Skene. See Mail matter.
-- Black's Law Dictionary
Legal Definition
A portable receptacle in which mail matter is conveyed by post. See 26 Fed. Cas. (U. S.) 803; 9 Am. Law Reg. 145.
-- Ballentine's Law Dictionary