The default action when deleting a message via the webmail interface is to move the message into the Trash folder. Messages must be deleted from the Trash folder before space is freed up from your mailbox account. The steps below explain how this can be done.
The deletion of messages from the Trash folder can be done automatically upon logging out of webmail. If you'd like to set this up, you can follow this guide.
Step By Step
Select the message you would like to delete by clicking it once to highlight it. You can highlight multiple messages by holding the shift key.
Click the "Delete" button on the toolbar near the top of the screen:
Click on the "Trash" folder in the list of folders in the left pane:
Click the button with the gear icon in the bottom left corner of the screen and select the "Empty" option to empty the trash folder: