This is mail that is either "Letter" or" Large Letter" format under 100g, under 5mm thick, clearly addressed with a full UK postcode on the last line of the address, using a font which can be read by "Optical Character Recognition" cameras, (ie Arial 10 point). It must be bendable, ie no solid objects or bound spines. The OCR engines can read handwritten mail, but there is no guarantee on this as there is such a wide variety in standards of individuals handwriting.