Not sure you understand how corporate email works...
The email sync doesn't keep the content of all your emails on your phone (at least not when used with a corporate email server such as Exchange). I ran 16 GB iPhones for years, filled them up and never, ever had a single issue with email.
You DO NOT need to delete emails from the phone. My mailbox on the server is many GB, and it is not stored on the phone. For most of my iPhone 3G - 4S lives, they had multiple GB free and were all 16 GB phones.
If you are attempting to use your phone as the single source for your email, you're making a huge mistake anyway. If the phone was to get damaged or lost, you're screwed.
I was referring to using the email mail app. If your cooperation requires an different app then it's up to how that s programmed. I have 4 different email accounts on my iPhone 5s using the mail app and according to my iPhone I'm I'm using over 5gb of data from all the downloaded emails. When i delete them from my phone it doesn't delete from the server but I have to delete in order to regain space.