Mail account at storage limit, can't find where all the storage went

Mail account at storage limit, can't find where all the storage went

One of our accounts says it is at limit, but I can't find what is taking up all the storage. I've cleaned out thousands of mails from the account, and it still shows to be at 101%...but when I manually click through all the folders, I can only account for about 30% of the limit.

Where can I find out what's causing the storage problems?