Standard Multiplayer world with friends, no mods.  I designed a mailbox system utilizing a scroll rack and a couple of trap doors, with an optional chiseled block and wrench to serve as a notification system.  This system doesn't compromise room insulation as long as you use solid trap doors, but slated doors could make the notification block unnecessary if you like.

  
Picture 1:  Outside of my room I keep a stock of parchment and an ink and quill to write with.  You can see the trap door that serves as the lid of the mailbox.

(Bonus:  Beneath the mailbox you can see the public records bookshelf I've been working on.  It has a book on the world's history, one on translocator information, and one on resource locations so that people can add relevant waypoints for areas they haven't been to yet.  One of the others has already started adding to the public record books with me.  This is the nerdiest fun I have ever had playing a multiplayer game.)

  
Picture 2:  The mailbox, opened up to receive a letter.  The chiseled block above has been hit with a wrench to rotate it and reveal the New Messages face.

Pictures 3 and 4:  Inside of my  room, with the other trap door set in its two positions.  I like that I don't have to let people peek into my room every time they drop off mail, and I love that I can stash a bookshelf behind the door but I haven't written anything to fill it yet because our leather is needed elsewhere.  (We're trying to get everyone outfitted with elk for travel!)

  
This has been a really fun way to find novel uses for stuff I've not gotten to play with much.  Multiplayer with friends is a totally different experience, and it comes with unique challenges as well as myriad benefits.  I love this game so, so much, and even the vanilla experience has provided so much more fun and opportunity for creativity than I had ever expected.  Feel free to brag about your own design innovations or any uses of books and parchment in your own worlds.

Vanilla Mailbox