Mail app crashes on iPhone 6
What eventually ended up solving the problem was removing my email account from the phone, then re-adding it.

The steps to doing this are:


Quit Mail (Double-click Home button, swipe up)
Open Settings
Tap Mail, Contacts, Calendars
Select your account, then hit Remove / Delete
Add it back by going back to Mail, Contacts, Calendars, selecting Add Account, and re-entering your information.