To delete a selected message or attachment, select Delete from the Edit menu, or click on the Trash button.
You can also choose to delete automatically any messages saved to a mail folder.
To restore the most recently deleted message or attachment, select UnDelete from the Edit menu. You can select UnDelete up to ten times to restore the ten most recently deleted items (most recent first).
To restore multiple messages or attachments, or to restore an item that was deleted before the last ten deletions, select Deleted from the Mailbox window View menu. A list of the messages deleted during this Mail session appears. This list includes multipart messages with one or more deleted attachments. Select the item or items you want to restore and select UnDelete from the Edit menu.
To restore all deleted items select Deleted from the Mailbox window View menu, select Select All from the Edit menu, and then select UnDelete.
Deleted messages are only saved until you leave Mail or until you discard all deleted messages. After that they are permanently gone.