Setup Office 365 email with built-in iOS mail app:
IMPORTANT: Delete Your Old Email Account with the same Email Address from your Phone (If you are migrating from another Email System)
1. Click the Settings Button on the Home screen
2. Select the Mail, Contacts, and Calendars option
3. Select your old Exchange Account
4. Press the red Delete Account button.
5. If prompted to remove contacts from your account, click “Delete from my iPhone”
Add Office365 account to iPhone:
1. Tap Settings > Accounts and Passwords > Accounts > Add account > Email. Tap the Exchange option.
2. Enter your full email address, for example email@example.com, and your password (If Requested), and then tap Next. Change the Description to your email address or company name.
3. On the next screen, tap Sign In
4. Enter your Account Password
5. If you see the following window, tap Accept
6. By default, Mail, Contacts, Calendar and Notes information are synchronized. Tap Save.
7. If you're prompted to create a passcode, tap Continue and type a numeric passcode. If you're prompted and don't set up a passcode, you can't view this account on your device. You can also set up a passcode later in your settings.