With iOS 11, the way that you configure your built-in Mail application changed. Follow these instructions to configure the built-in Mail app for iOS 11.
1. Go to your device's Settings.
2. Find Accounts and Passwords.
3. Select Add Account.
4. Select Exchange.
5. Enter your email and a description for this new account.
6. Select to Sign-in to the exchange account.
7. Enter your computer logon password, and click Sign in.
8. Click Accept.
9. Select the items that you want to use, and click Save.
10. The Outlook account or the description that you chose has been configured. Just exit the Settings application, and go to the Mail application.
11. Select Outlook or the description that you entered to open your new account.
Note: If you have iOS 10, follow these instructions: