Return Mail

I'm trying to find a precise definition of the phrase "Return Mail" (as in "please reply by return mail"). I'm particularly interested in whether this connotes that the reply should be prompt. Would a reply posted 18 months later still be a reply by "return mail"? Any help would be most appreciated.

I'd go beyond prompt... to me, "return mail" connotes an immediate reply, within a day or so. 18 months is certainly unacceptable.