In a folder "owned" by an account -- say, you open Inbox or such -- the delete icon matches the account's "delete method"
X for "delete immediately"
Trash can for "move to deleted"
A small crossed over "eye" for "move to hidden" (my lame attempt at representing "hide from view")
In the smart inbox, this icon will delete each selected message with its account's specific "delete method".
However, the icon is a generic "trash can". I don't happen to have one to represent "smart delete" (trash can with a brain on top? headcrab style?)