The title is actually MAILMAN, BRING ME NO MORE BLUES. It was recorded in 1957 by Buddy Holly, and written by Ruth Roberts, Bill Katz, and Stanley Clayton. ("Stanley Clayton" was a pseudonym for Bob Thiele.)

It was also covered by the Beatles and by Lefty Frizzell.

The phrase given above as "It's certain, Mailman" is heard by some people as "Mister Mailman" or "Yes, sir, Mailman." Also, some people hear "one little letter" instead of "one blue letter."