Most people have their mail set to Push, which automatically sends new data whenever it is available. That means that your phone is constantly polling your email provider’s servers to see if anything new has come in. This will absolutely destroy your iPhone’s battery. Go to Settings > Mail, Contacts, Calendars > Fetch New Data and toggle off the Push option. It’s important to make sure that you set all of your apps to Fetch, then adjust the Fetch schedule—the less often you Fetch, the more battery life you save.