In England, men and horses were at one time stationed at intervals on a few major roads. Riders carried the official messages, or "king's packet," from one post to the next. Since messages were relayed from post to post, the system became known as the "postal service."
But what about the packet? Letters were carried in a leather pouch known as a "male," from Old French. A carrier carried "a male of letters." In time, the spelling was changed, so as not to confuse it with the male gender. A term for any bag of letters became "mail," as well as the letters themselves.
One popular theory has it that this phrase for a regular or favorite gathering place derives from male prairie chickens. The birds used to congregate on knolls in the spring and perform such elaborate courtship dances they'd stamp the hills bare.
Throughout Indiana and Illinois, many such places were made bald by these birds, as well as by the stamping of horses, bison, and cattle congregating to "set turf." Americans still sometimes use the term to mean an old courtship location.
SOURCES: 'Morris Dictionary of Word and Phrase Origins,' by W. and M. Morris; 'Why You Say It,' by Webb Garrison; 'Loose Cannons and Red Herrings,' by Robert Claiborne; 'Have A Nice Day!' by Christine Ammer'; 'Dictionary of Word Origins,' by Jordan Almond; 'Encyclopedia of Word and Phrase Origins,' by Robert Hendrickson.