The first way
Right-click the message, then select Delete from the drop-down menu.
The second way
Tick the checkbox corresponding to the message you want to delete, then select 
on the Action bar.
The third way
Drag the message in the mails list pane and drop it into the Trash folder in the left pane.
All temporarily deleted messages will be in the Trash folder.
Note
All messages which have been deleted permanently cannot be restored any longer. To delete a message permanently, the message should first be in the Trash folder. It means that the message should have been deleted temporarily.
1. Select the Trash folder in the left pane to see all messages in this folder.
2. Right-click the relevant message, then select Delete from the drop-down menu;
Or, tick the checkbox corresponding to the relevant message, and click 
on the action bar.
Note
This function is only for messages which have been TEMPORARILY deleted.
The first way
1. Select the Trash folder in the left pane to see the list of temporarily deleted messages in the right mails list pane.
2. Tick the checkbox corresponding to the message you want to undelete.
3. Drag and drop the message into another folder.
The second way
Open the Trash folder, then use the Move function to remove the message from the Trash folder.