Pretty cool, but whay are some envelopes transparent.
Come to think of it, it does add a sense of motion. Alternatively you could have used motion blur, but I like your way too.
I agree Newsy... solid would look better.
If you are trying to create a motion effect it would be better to leave the envelopes in a solid format but apply a tapered motion trail from them to the mail slot opening. For added depth, apply drop shadows from each envelope with varying offsets and opacities. This would make it seem like the envelopes are flying out of the box.