Hacked

Go to Settings. Scroll to the bottom. Look just below VPN and you should see either “Profiles” or “Device Management”. Tap on it and you will see a profile. Tap on the profile and delete it. You can then delete the email account.

Note that if it is there someone installed it; profiles don’t appear on their own out of the thin air. If you have a company, association or school account on the phone it may have been installed as part of setting up that account. Otherwise, you authorized installing it, possibly in response to a “free” offer.

if you didn't jailbreak your phone, it's not hacked.

did you buy this phone used from someone? if so, the phone wasn't properly reset and you may have other issues

do you share an Apple ID with someone? if so, you should not

if you are in Settings > Mail > Accounts, you should be able to delete an account by tapping on it and selecting Delete Account