iPhone 4 indicates 6.1 GB used by email: Not True. How Fix?

iPhone 4, iOS 7.1.2, 16 GB storage capacity

My 16 GB iPhone says I have 6.1 GB of usage consumed by email. This number only ever increases, even after I've gone into my mail accounts and deleted most of the emails, especially any with videos, pictures or attachments. I finally deleted the email account from my phone (it was an IMAP account), as I assumed that was likely causing the problem. Even after I did that, there was no reduction in the Mail Usage. My other email is a POP account.

When I initially brought it into an Apple store to see if they could fix it, the guy said that I might need to back everything up, restore the phone to the factory settings, and then restore my files. He also said the problem might still be there if it was included in the backup. He also said that, if I got a new phone without fixing the problem, it would carry over to the new phone!! But he didn't have a definite fix for it.

A friend said she had had the same thing happen, but when she brought hers into the store, the guy was able to go in and clear/delete all of these files that didn't actually show up in the mail account, but that were still being stored and counted in the usage. My phone works except for the fact that the usage is pressed up against the limits, resulting in the Home Button not functioning.

Has anyone else dealt with this? Do you know how to fix it? Thanks for any help.