Emails not deleted from POP accounts on iPhone even after emptying trash, taking up lots of space

iPhone 5, iOS 7.1.2 (but issue has existed for multiple updates)

I have 3 separate POP accts. on my iPhone (please, no "why not use IMAP?" questions, that's a separate discussion), all 3 are set to "never" delete email from the server because on my main desktop (also using Mail) I have the same POP accounts set to download and then delete all email from the server. I typically don't keep ANY email on my iPhone, I only use it to check email when away from my main computer. This system works well for me except for the following...

The problem is that even though I manually delete AND empty the trash in all 3 POP accounts on my iPhone, if I look in settings (General > Usage), it shows that Mail is using 1.3 GB of space/memory. I've watched this grow as it has resisted all attempts at finding a way to actually delete the emails from the iPhone.

In addition, if I use the iPhone's search feature, I can find emails (even drafts that were never sent) that I deleted a long time ago. When tapping on a search "result" for one of these supposedly deleted emails, it opens Mail on the phone but only says "No Mail". The iPhone has gone through multiple sync's with iTunes and upgrades since I first noticed this.

So, how to actually get those old emails deleted from the iPhone and recover the 1.3 GB of space that I need?

BTW, I did Google this issue before posting this and came across many similar questions but no definitive answer to my exact issue, as most of the similar questions involved people not understanding the difference between POP and IMAP accounts, or just not having a setting correct on their iPhone, etc...

Thanks, I appreciate any help.