You go to the post office to mail a letter. Your mail is delivered by a postal worker, who works for the Post Office, and might drive a mail truck. There are publications called things like The New York Post, others called The Daily Mail, and some that cover all the bases like The Post and Mail.
So what’s going on here; why do we keep using these two different words more or less interchangeably?
As it happens, using post and mail to mean the same thing is pretty recent; they originally referred to different things. “Mail” comes from Old English, where was borrowed from the Old Scandanavian word “mál” (discussion or agreement). In Old English, and for quite a while afterward, “mail” meant a deal or a contract. One of the most common kinds of contracts was how much rent you ought to pay, and before long “mail” took on that meaning — it meant rent. In those days the monarchy owned all the land, and “mail” came to mean the other payments made to the ruler: taxes and tributes.
“Mail” isn’t used to mean rent very often nowadays, but it was still current in 1900: “He carried a great sum about with him, being the rents and mails of all his New Milns property,” and has even appeared as recently as the 1960s.
You might notice that although “mail” has a very long history, it doesn’t include any mention of letters, correspondence, or sending things to someone by way of an intermediary. That’s because the “mail” that does involve those things is a completely different word. The “mail” that’s similar to “post” arrived in English just about as long ago, and this one comes from French, where it was “malle.” It meant a container that you might use while traveling. A case, bag, wallet, or really anything like that.
Although people certainly sent letters before then, it wasn’t until the 1600s that the idea of having a whole bag full of nothing but letters appeared. At first, that sort of thing was called a “mail of letters.” By the later years of the 1600s you didn’t have to specify “of letters” any more, but a “mail” was still a singular bag or box, so it was used like this: “Our Pacquet-Boats put to Sea yesterday with the Mails for Calais.” It took until the 1800s for “mail” to refer to what was in the bag: “He walks as if he had the missing mail in his pocket and an extra to issue immediately.” (1844).
As you might expect, there are a lot of different meanings for “post,” from a career assignment to the vertical hunk of wood that holds up a fence. A post can also be used as a marker, like a “milepost” that tells you how far along a road you’ve gone. That’s the version that acquired the same meaning as “mail.” The mileposts enabled people figure out where along the road to station horses and riders to take over in a relay when somebody had to send a message as quickly as possible. That’s how the Pony Express worked in the American west, but the system itself is a lot older.
The people who carried the dispatches were called “post riders”, because they rode from one post marker to another. The system worked pretty well, and became a fairly regular service in some parts of Europe as early as the 1600s. Somebody figured out that if the post rider wasn’t in when you came in to send your letter, you could just leave it there until the next rider arrived — and for safekeeping, you could put your letter in a box. That is, a post box (which could have been called a mail, but evidently nobody thought of that).
As the system became more widely distributed, it started to make sense to have some extra people doing things like organizing where each post rider would travel and when, and sorting the letters in advance. Those jobs were done in a post office. And the post riders? Just like today, what they carried was mail — and technically they even carried it in a mail.
