LOL..It was never adopted indeed.
I found this on the motto:
Sometimes considered the motto of the United States Postal Service, even used to refer to the Pony Express rider... this is in fact a translation of a line written in the fifth century BCE by Herodotus, the Greek writer known as “the father of history.” His master work was On the Persian Wars composed between 500 and 479 BCE. In this famous passage, Herodotus praises the stamina and persistence of horsed messengers in the service of Xerxes, king of Persia.
Interesting historical find—from Herodotus, no less. Who’da thunk it?