To ensure prompt receipt and security of mails, it is necessary to install a suitable and secure letter box which is accessible to delivery staff. For details, please refer to the link below:
http://www.hongkongpost.hk/doc/misc/letterBox/install_letterBoxes.pdf#page=1
Standard A4-sized stainless letter boxes are available for sale at Hongkong Post. For details, please refer to the link below:
http://www.hongkongpost.hk/eng/services/supplementary/index.htm#letterbox