Thanks very much for the link. Rather silly of me not to associate the post (of letters) with the post of horses, probably with the same stages and stations.
What threw me off, I guess, was Dumas using the expression "jeta la lettre à la poste" rather than "mit la lettre ..." "Jeta la lettre ..." (threw the letter in the mail) gave me the impression that the novelty of the service had already worn off; that it was already common occurrence which somehow I couldn't associate with 1651. Now I see, from your link, that indeed the service was already some 50 years old by 1651. Thanks again.
And since we all like images:
