The Mexican postal system has its roots in the Aztec system of messengers which the Spanish adopted after the Conquest. A postal service was established in 1580, mainly to communicate between the viceroyalty of New Spain with the motherland Spain. During the 18th century, Spain established a formal postal system with regular routes. In 1856, Mexico issued its first adhesive postage stamps, with "district overprints", a unique feature among postal systems worldwide, employed to protect from theft of postage stamps. In 1891, the postal and stamp issuing authority was created as an administrative division of the Secretaría de Comunicaciones (Secretariat of Communications). It was called Servicio Postal Mexicano (Sepomex). In 1901, the Dirección General de Correos (General Direction of Mail) was made a separate government agency. The Palacio de Correos de Mexico is used since 1907 as main post office. The Mexican Revolution and ensuing Civil Wars (1910–1920) resulted in numerous provisional and local stamps issued by the factions in control of different areas of the country. Colonial period The postal system of Mexico may be said to have begun with the Aztecs, who operated a system of messengers; they worked well enough that Hernán Cortés continued using them after the 1521 conquest. After 1579, the right to operate the posts was farmed out to members of the nobility, who were known as "Correo Mayor de la Nueva España". The most important part of their operation was the route between Mexico City and Veracruz. In 1742, the administrator of posts in Madrid was ordered to improve the Mexican system, resulting in the 1745 establishment of a weekly post between Mexico City and Oaxaca, followed in 1748 by a monthly service to Guatemala. In 1765 the Spanish crown bought back the rights to the postal service, effectively "nationalizing" the posts. During the Colonial period and continuing up to the introduction of adhesive stamps, letters were typically sent collect, the postage to be paid by the recipient upon arrival. The covers, or envelopes in which the letters were sent, were stamped by hand with the name of the originating town, and typically with a number representing the charge for postage, e.g., "3" for 3 reales. Occasionally, mail was sent postage pre-paid, in which case the envelope would be marked "Franca" or "Franco" or "Franqueado." According to Yag & Bash (1965), some handstamps (on undated covers) date to the 1720s and the earliest