i've deleted ALL my mail and messages yet my usage still shows that i have 3.4GB of storage being taken up.

I'm trying to update my Iphone 5 to iOS7.1 but it tells me i do not have enough storage space. i went into usage settings and it said i was using up 3.4GB of storage in mail, so i deleted ALL of my mail and it still says that i'm using 3.4GB of space? What gives? I've deleated all the inbox, sent, and trash folders yet no change?