1
general::
Phrase(s): by return mail [and] by return post
by a subsequent mailing (back to the sender). (A phrase indicating that an answer is expected very soon, by mail.) • Since this bill is overdue, would you kindly send us your check by return mail? • I answered your request by return post over a year ago. Please check your records.
McGrawhill's American Idioms And Phrasal Verbs