One who rides over a postroad to carry the mails.(noun)
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Use "postrider" in a sentence
"All right, Mr. Clarke, I've got to have a postrider to carry a very important message to Dover, Delaware, to get a man back here from Dover by nine o'clock tomorrow morning."
"Yes, yes, it's Rodney -- Rodney and the postrider -- they're coming!"
"Now the postrider was to the people of Revolutionary days what the telegraph or the telephone is to us today."