Most of the data is stored as cache on the device. You can try using a cache clearing app to reduce the amount of space mail takes up. This comes with a free 3 scan trial.
https://itunes.apple.com/us/app/memory-disk-scanner-pro-check-system-information/id921515621?mt=8
Thanks. I am looking at the app but it appears to only search not remove data?
The problem is that I'm having some weird issues with my email account and it randomly redownloaded all my emais and managed to double the amount of data stored on iPad.
By removing all data and rebuilding my mail I can correct the mail issues as well as regain the wasted space.
I even tried plugging iPad into Mac and using Iexplorer app but unless the iPad is jail broken I can't access OS related data.
It will remove the data, just make sure you do the Disk scan. Or you can use this app. This one works just as well:
https://itunes.apple.com/us/app/dr-cleaner-phone-clean-clean-similar-photos/id1156773866?mt=8
But that is very odd that your email account is automatically redownloading emails. I have not experienced that, so I'm not sure how to help with that issue. Sorry!