Hey, thanks for the submission. With the big caveat that I haven’t actually played the game, just read it, here’s my review.
Theme
Right out of the gate I'd like to say - I love the idea here! I love the idea of the letter and map travelling. However, unfortunately I don't think it actually addresses the the theme of the jam, as there is just one map. It's true that different people will contribute to it, but in my reading the jam theme requires multiple maps of the same place/space, varying in scope and/or scale. This creates a map of one place.
Maps
The map is basically the product of the game. It is central to the game, but as I've mentioned in other jam submission comments, I think the map is mostly a product or artifact of play rather than the space of play itself.
Tone and style
I like that this is simple and to the point. The layout is great.
Elegant
I like the simple and light mechanics. I wouldn't feel intimidated opening the letter, but rather would want to participate immediately.
Easy to understand
Unfortunately as it says on the jam page, this still needs a few more instructions, particularly around getting started. That's too bad, because I feel like it's super close to a playable game! Please finish it! I'll probably start a letter even if you don't, but I'd love it if it were finished.
So, in conclusion, I really like this one, I just don’t think it addresses the theme of the jam very closely. But in the end it's still a map game, and I'd like to play it!