You may have created another Trash folder to hold all your deleted email messages. This can happen when you use another email client together with Evolution.

Evolution's Trash folder is a Search folder. It does not really exist but simply displays all the messages that are marked for deletion in any folders of that account.

You can easily tell the two Trash folders apart by looking at their icons. The Evolution Trash folder has a special icon whereas the other physical Trash folder looks like any other folder.