Mail.app shows three bins: archive, trash and junk. I find it extremely confusing because they all are essentially bins. The icons fail their main function: being easily identifiable among its neighbours. If you look for a metaphor for you icon, consider what it will be standing next to.
In this case I’d prefer a checkmark (Done) for archive and a cross (maybe?) for deletion. Spam should probably just be a word, it’s hard to find a good metaphor for it.