I want to retrieve a deleted mail
Because deleted junk mail does not go to the Trash folder, it is not possible to undelete it. The best policy is to leave junk where it is. This may vary with email providers, but generally the junk is auto-deleted after some time so it does not take too much of your server space.
On local folders, deleted junk similarly does not go to Trash, and undelete is also not possible.