A reply by return mail

A lovely phrase!

"A reply by return mail" is an old-fashioned way of saying that someone will respond to a letter or message by sending a reply back through the postal service, usually within a short period of time.

In the past, when people wrote letters to each other, they would often include a self-addressed stamped envelope (SASE) with their letter, so that the recipient could easily send a reply back. This was especially common for business correspondence, where a prompt response was important.

The phrase "a reply by return mail" is still used today, although it's less common now that we have email, text messaging, and other digital forms of communication. However, it's still a charming way to express the idea of a quick and courteous response.