Mail App take nearly 1gb of Iphone 5s but nothing in mail app

Hi, I got a problem with my Mail app. I use an Iphone 5S (16gb)

I check Usage in Settings and found that Mail and Attachment is nearly 1Gb of my phone. But actually in my Mail App I did delele everything. I have only one email account added.

Although I delete this account then restart my phone, the situation is still there. I do not know what to do because the capacity left in my phone is only more than 40Mb. Some app need a free space at least 100Mb.

Can anybody help me?

Thanks alot.