Access your e-mail on iPhone - iOS Mail App
STEP 1:
Go to Settings > Apps > Mail, then tap Mail Accounts.

STEP 2:
Select Microsoft 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.
STEP 4:
Enter the password associated with your email account. Tap Sign in or Next.

STEP 5:
The Mail app may request certain permissions. Tap Accept.
STEP 6:
Choose the services you want to sync with your iOS device and tap Save. You're done!

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