Actually my email is a POP account.
Here is what my iPhone says is the itemized storage consumption for each app:
Mail 185.6 MB
Photos 82.8
Dictionary 75.3
Airline App 57.5
Shazam 57.5
Banking App 45.6
Local Bus App 44.4
Tune In Radio 40.4
Transit 26.7
Messages 11.5
Contacts 2.8
Calendars 1.8
iBooks 0.537
Health 0.451
Safari 0.164
--------------------------
Total 633.052 MB
The total at the bottom is my own addition. However, the iPhone says that total used storage is 12.15 GB, and only 652.6 MB is available. So, something is using an enormous hunk of storage that the phone isn't able to display.
Edit:
Forgot to mention that I did reset the phone when the storage didn't get freed up, but it didn't change anything. So, it looks like I'll have to do a factory reset.