How to delete storage in my mail account on iPad air2? It is 7 gb and I cannot see how to delete it

[iMore Question]

I have a 64 gb. Ipad air2 . When I check my mail storage to manage it, it indicates 7.3gb used!! Where is this and how do I delete it? Trash, archive and current emails all cleared off but storage still indicates 7.3 gb used! No one at stores where purchased can help. (Apple sales staff!) It is a new problem for them!
Any solution much appreciated.
Thanks.
Jonhnac