Known Issues: Horde and Squirrelmail privacy problem
From CPWiki
[edit] Privacy warning
Horde and Squirrelmail allow any user of same account (domain) add other emails on folder list, so it can view any other email content on same domain.
We could patch Horde and Squirrelmail to fix this issue, but, since cPanel will auto update these scripts ocasionally, a patch is not an option.
[edit] How to fix it
Remove Horde and Squirrelmail from cPanel WHM -> Tweak Settings
