Great game! The rules are easy to understand, but you really got a lot of mileage out of them; the variety of puzzles is really impressive. Some of them really had me sitting and scratching my head for a while, and for many of those, there was a really nice "a-ha" moment when figuring out what I was meant to do. Art, music, and sound all complement each other well; the little mailpeople are so cute!
Really just a great game all-around, and super impressive to have done it all from scratch in your own engine, too. Great work!