The only solution unfortunately may be to delete your email account and add it again. Its ridiculous, but the email app persistently seems to cache emails even if you get rid of them. you could also use another email app like acompli. for more information, you may want to read this:
On 01/10/2016 01:53, Henry Miller wrote:
Trying to create more free space on my IPhone 5S 32 GB running the latest I O S 10.0.2. I noticed that Mail was using 4.3 GB of space and I deleted all emails messages in all folders. However, When I go to setting, storage and ICloud usage, and then manage
storage, I discover that mail is still using 4.3 GB. This is despite
switching off the phone and restarting. In addition, the amount of free space on the IPhone is slowly diminishing, obviously there is data being accumulated but I don't know where to search. Facebook is 1.4 GB and this also needs to be reduced in size.
Please can anyone offer some suggestions?