A recent cPanel/Horde update has changed the default setting for this option to off.
To enable it again, just take the following steps -
- Log into webmail.yourdomain.com using your cPanel username and password
- Click on the "Preferences" icon (looks like a gear or cog) in the top menu
- Hold your mouse over the word "Preferences" and then click the "Mail" sub menu item that appears to the right.
- Click "Folder Display" (the last option on the far right)
- Untick the checkbox beside "Use IMAP mailbox subscriptions?"
- Click the "Save" button.
When you return to the "Mail" page all of your mailboxes will be visible and accessible.