This paraphrases the motto of the US postal service: “Neither snow nor rain nor heat nor gloom of night stay these couriers from the swift completion of their appointed rounds”.
In Tom Burnam’s More Misinformation it is explained that this quote by Herodotus is not really the official motto of the Postal service, since there is no such thing. But it is a quote that is inscribed on the General Post Office building in New York, and has been construed as a motto by the general populace. It refers to a system of mounted postal couriers used by the Persians when the Greeks attacked Persia, around 500 BC.