One of the ideas my collaborator and I didn't go for this jam with was kind of the opposite of this, where you play as a fallen Nigerian prince and have to send as much spam as possible to restore your kingdom. This is well developed, great soundtrack too. Godot, represent! Our games have something in common huh, utilizing the whole keyboard?
Damn this gets confusing though. Like @Capital Ex, I also have to press FN + f# key to switch boxes, which I think makes it too hard for me. That moment when the difficulty spikes and more come in quick = game over for me every time. I want it to go on for longer before I lose, but that might just be my laptop keyboard setup at fault. IMO, arrow keys would definitely be better. It should also more clearly indicate which inbox you have currently selected, I didn't notice the color bar until my fifth time through. Many of the characters are really tough to read at a glance as they pop out of the spinning envelopes. 0 vs. O for example. l vs I vs 1 vs 7. M vs. W. B vs. 8. 9 vs 6. These could either be cut entirely or made very distinct; although I see that's where part of the challenge come from, I didn't personally find that enjoyable. I think if there was little "shiiiick" sound effect that played while the mail slid over to its appropriate category, that would add something to the responsiveness feel and positive feedback.
This is a unique and original entry, which is my favorite kind to see. Well done for taking a bright idea and running with it as far as you could in the time allotted.