Letter boxes / plates should be considered as an aperture which can be used by the criminal, either to extract goods from within in close proximity to the door, commonly door keys, or to work vulnerable locking devices from the inside. Letter plates should conform to British Standard.

They should be positioned not closer then 400 mm from the door lock and under no circumstances should they be fitted to the bottom rail of the door. An internal cover plate offers additional security, as does a letter basket, though you might wish to consider removing the bottom, allowing the mail to fall to the floor and thus preventing theft of the mail.