On October 9, 1874, a treaty was signed in Bern to establish a Universal Postal Union. It is this day that the whole world now marks as World Mail Day.

The number of member states in the union increased so rapidly over the next three years that its name was changed to Universal Postal Union in 1878. In 1947, the Air Force was granted the status of a specialized UN intergovernmental organization.

On July 1, 1879, Bulgaria became a full member of the Universal Postal Union. In 1880, the Principality received the first invitation to a conference in Paris and this was the beginning of the official international contacts of the Bulgarian Post.

International Letter Week is commemorated by the decision of the 14th Congress of the Universal Postal Union in Ottawa, Canada, in 1957, the calendar week that includes the date of October 9 - World Postal Day.