Blank envelopes are crafted with 6 paper. Punch with a blank envelope to open the letter-writing formspec, which allows you to specify an addressee, letter text, and an optional "Attn." field which will be displayed in the envelope description. Once writing is complete, the envelope is sealed by pressing the "Seal" button. The blank envelope is then replaced by a sealed envelope which shows the sender, addressee, and optionally the Attn. field in the description. If the addressee punches with the sealed envelope, it will become an opened envelope, which, once punched, shows a formspec with the text. While a sealed envelope can only be opened by the addressee, an opened envelope can be read by anyone - thus, secret correspondence should be pulverized!
Soft dependency on default for crafting.