You have likely installed a profile.
Follow these steps to remove it:
To remove the profile - including the associated email account:
- Go to: Settings > General > Profiles & Device Management (or Profiles, depending on your iOS version) > tap on the configuration profile > tap on Delete Profile.
To delete the associated app:
- Touch and hold the app icon until it jiggles, then tap on the "X" that appears > press the Home button when done.
Restart your iPhone (normally):