When was the first mail delivered via the Pony Express

The Pony Express was a mail service delivering messages, newspapers, mail, and small packages from St. Joseph, Missouri, across the Great Plains, over the Rocky Mountains and the Sierra Nevada to Sacramento, California, by horseback, using a series of relay stations. Officially operating as the Leavenworth and Pike’s Peak Express Company of 1859, which in 1860 became the Central Overland California and Pikes Peak Express Company, this firm was founded by William H. Russell, Alexander Majors, and William B. Waddell all of whom were notable in the freighting business. During its 18 months of operation, it reduced the time for messages to travel between the Atlantic and Pacific coasts to about 10 days. From April 3, 1860, to October 1861, it became the West’s most direct means of east–west communication before the telegraph was established and was vital for tying the new state of California with the rest of the United States.