When you delete messages and contacts, the file is moved to the Trash folder. You can recover files from the Trash by dragging and dropping them into another folder.
Messages left in the Trash for too long may be automatically purged, in which case you cannot recover them.
To remove a message from Trash
Open the Trash folder and select the message to remove from Trash.
Drag the message to one of the folders in your Overview Pane. You cannot drag messages to the Drafts folder.