by return mail and by return post

by return mail

Through mail correspondence back to the sender. Please indicate your interest in this offer by return mail.
See also: by, mail, return

by return post

Through mail correspondence back to the sender. Please indicate your interest in this offer by return post.
See also: by, post, return

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.
See also: and, by, mail, post, return