More or less the same idea of yours, but also two well differentiated approaches. Who would have thought this could happen with that theme?
At first, the idea was that each envelope would have its own bar moving and you can only press that key on certain areas of the bar. This spammed the screen with rotating bars all around and made it crazy hard to play, so we went with categories. Much easier, just focus on classify as many envelopes as possible from the same category at once and then change to another one.
Confusion between look-alike characters was also intended, a pun to how spam tries to confound us with subjects and mail names. What may be more annoying is the constant open/close animation of the letters, this could be fixed in a following iteration.
I think I might add difficulty settings at start or make the curve smoother (this curve controls how much time elapsed between envelopes).
Sounds also play on a successful classification, but only when the envelope reaches destination. Now that I think of it it was not the best decision, will think something different.
Thanks for the detailed review!
P.S. Godot for the win!!