In order to delete a mailbox, you will firstly have to mark the checkbox next to the name of the mailbox. If you want to delete several maiboxes at one time, mark their respective checkboxes.
Your next step is to use the “Delete” button, located above the list of created email accounts. A new window will pop, asking you if you are sure you want to delete all of the selected mailboxes. Click “OK” the delete them, or cancel to leave them active.