STEP 1:                   

Go to your device's Settings, scroll down and tap Mail > Accounts >Add Account.


Device Settings > Mail 


Add Account 


STEP 2:                   
Select Exchange.


Choose Exchange 



STEP 3: 

Enter your Office 365 email address, password, and a description (Optional) of your account. Tap Next.


Note: Your email address should be your Office 365 business account or school account. Such as @yourdomain.com, @domain.onmicrosoft.com, or @schoolname.edu.


Exchange Sign In 


STEP 4:                   
If you're prompted to enter server settings, enter the following and tap Next.

  • Email: your full email address.

  • Server: outlook.office365.com.

  • Username: your full email address.

  • Password: email account password.


STEP 5:                   
The Mail app may request certain permissions such as accessing your calendar and contacts. To agree, tap Yes.

Then choose the apps you want your email account to sync with your iOS device and tap Save.


STEP 6:                   
Now you're all set to go!       


Source.