Internal mail envelopes are envelopes designed specifically for hard-copy communication and document transfer within organisations. As such, they are reusable and distinctively coded to differentiate them from external mail. Multiple addressee boxes make multiple mailings possible: each time a new mailing is made, the sender crosses out the previous addressee on the internal mail envelope and writes in a new one. Tuck-in flaps or string-and-washer seals are reusable. Punched holes confirm or otherwise the presence of documents inside.