Mail, or post, is a method for transmitting information and tangible objects, wherein written documents, typically enclosed in envelopes, and also small packages, are delivered to destinations around the world. Anything sent through the postal system is called mail or post.

In principle, a postal service can be private or public. Governments often place restrictions on private postal delivery 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.

Postal systems often have functions other than sending letters. In some countries, the postal system also has some authority over telephone and telegraph systems. In others, postal systems allow for savings accounts and handling applications for passports.

Early postal systems

Persia (Iran)

The first credible claim for the development of a real postal system comes from Persia (present day Iran) but the point of invention remains in question. The best documented claim (Xenophon) attributes the invention to the Persian King Cyrus the Great (550 BC), while other writers credit his successor Darius I of Persia (521 BC). Other sources claim much earlier dates for an Assyrian postal system, with credit given to Hammurabi (1700 BC) and Sargon II (722 BC). Mail may not have been the primary mission of this postal service, however. The role of the system as an intelligence gathering apparatus is well documented, and the service was (later) called angariae, a term that in time turned to indicate a tax system. The Old Testament (Esther, VIII) makes mention of this system: Ahasuerus, king of Medes, used couriers for communicating his decisions.

The Persian system worked on stations (called Chapar-Khane), where the message carrier (called Chapar) would ride to the next post, whereupon he would swap his horse with a fresh one, for maximum performance and delivery speed. Herodotus described the system in this way: "It is said that as many days as there are in the whole journey, so many are the men and horses that stand along the road, each horse and man at the interval of a day's journey; and these are stayed neither by snow nor rain nor heat nor darkness from accomplishing their appointed course with all speed".

South Asia

Systems for collecting information and revenue data from the provinces are mentioned in Chanakya's Arthashastra (ca. 3rd century BC).