a rigid file which opens like a box, usually made of strong cardboard and able to hold a large quantity of documents. BOX is a file extension for an email mailbox folder file format. Sri Vyshnavi Multi Products - Offering Box Files, Filing Boxes, Document Boxes, Document Storage Boxes, फाइल बॉक्सेज, Office Files in Chintal,. What is a BOX file? Mailbox format used by various e-mail programs; stores collections of e-mail messages that can be opened individually; replaced by.