Mechanical sorting is suitable for items meeting the requirements set for dimensions, weight, shape, colour and contents of machine sortable items. The recipient's address must be printed on the envelope or visible inside a window-panel envelope.
Items may not contain any hard or inflexible objects such as paper clips, keys or casings. Only sealed envelopes or cards can be sorted mechanically. Mechanically sortable items are posted packed in boxes, large items separate from small ones, with the address information showing in the same direction.