Greenblatt's poem

I'm reading The Big Secret for the Small Investor, by Joel Greenblatt. I love his witty comments about the post office:

Emblazoned across an entire city block on front of the building is the well-known motto "Neither snow not rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed arounds"

Later, he notes:

Of course, during a recent snowstorm it turns out that it's "only a poem", according to our local postmaster.

A footnote states:

Actually, it's a poem written by a famous Greek historian 2,500 years ago (but only delivered in the last few centuries, due to some unforeseen weather conditions.