Email Usage not reducing after deletion

Hello. I am having space issues on my iPhone 4s. I have 3 POP email accounts set up and had several gb showing up under Settings, General, Usage Mail.

I 1st deleted all the emails from each Inbox and also went into each email account and selected all the emails in the Trash and deleted them and no change in the emails Usage.

Only after I deleted each email account off the phone and re-installed it, did I get me space back. It has been a week now and I am back up to 322mb (see attached) even after I have been clearing out all emails at the end of each day.

Does anyone know how to clear off email space or is there some app to purge the Mail on the iPhone 4s?