There actually is a Mail tab for the enduser case detail window (WCASEU03). You can add the WEB tab WCASEUM to show the mails online. But like the other users, we don’t use that option.
If you really want to use the feature, you should consider setting a filter to BMA search view WCAS00EU, e.g. to show only inbound mails, only mails where BMA_TO or BMA_FROM or BMA_CC contain the customer’s mail address. That would enable you to hide mails you don’t want him/her to see.
(Note: That will work for POB 21.14. I don’t have instantly in mind when the new mail tab came up…)