I have done some initial research on this and coming up blank.
We have email configured with hosted exchange 2007 via AppRiver.
A user can see all his self-created folders in OWA and on his iPhone, but not in Outlook 2013. To make matters more interesting, the emails themselves still show up in outlook when you search, and even show the correct folder name in the grouping, but the folder itself is not visible.
What I have tried to far - searching for an email in the missing folder, openning it, doing and advanced search and then clicking "browse" which brings up the folder tree, but still doesn't show the folder in question.
I have also created a second profile on his laptop to pull down a fresh copy from exchange, but still no luck.
Help is urgently appreciated.