The phrase “right as the mail” is an idiomatic expression that conveys the idea of something being very reliable, accurate, or prompt. It suggests that just as mail is expected to be delivered reliably and on time, so too is the subject being referred to dependable or correct.
The expression may be used in various contexts, such as confirming the accuracy of information or the punctuality of an action. For example, if someone confirms a schedule, one might say, “You can count on it—it’s right as the mail.”
Overall, the phrase emphasizes trustworthiness and reliability, drawing on the conventional expectations of postal services.