I have removed all mail accounts from my iPhone and reset it. What's using storage?
I struggled with this and spent a whole day trying to figure out why my mail app on my iphone suddenly took up 2.5.gigabytes of storage!  I did all of the suggestions on various websites to no avail.  I did however finally FIX IT.  I deleted all of my mail accounts and then DELTED THE MAIL APP.  Presto!  This seemed to be the key to fix the problem.  I was able to recover 2.5 gigs of storage back to my 16gb iphone.   I then reinstalled the mail app and started over again.  This seemed to work for me.  Another thing to try is to download one of the many (and good) third party email apps.  Hope this helps you all.
-Patty