I have removed all mail accounts from my iPhone and reset it. What's using storage? I have deleted all my e-mail accounts from Mail from the iPhone -- iCloud, my work account, etc., and have reset the phone (powering off and then back on) a couple of times. 

In Settings/General/Storage, I still show 186 MB of data being used by mail. Is the Mail app itself 186 MB? If not, how can I free up some of the space being "used" when there are no accounts at all on the phone?
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