1. On your iOS device, go to Settings > Mail > Add Account.
  2. Select "Other" from the list of types.
  3. iOS Shared Office 365 Mailbox Step 1 
  4. Select "Add Mail Account"

    iOS Shared Office 365 Mailbox Step 2
  5. Enter a name for the mail account (e.g. "Enquiries").
  6. Enter the e-mail address of the Shared Mailbox.
  7. Enter the password of a mailbox that has access to the shared mailbox (e.g., your own password, if you have access to the shared mailbox)
  8. Enter a description for the mailbox.
  9. Tap "Next".
  10. Select "IMAP".
    iOS Shared Office 365 Mailbox Step 3
    1. Fill in the blank information:

      iOS Shared Office 365 Mailbox Step 4

      i) Host name: outlook.office365.com

      ii) In the user name box, enter the e-mail address of the account that has access to the shared mailbox, followed by a "\" and then the email address of the shared mailbox itself. (eg "your.name@yourdomain.com\enquiries@yourdomain.com")

      iii) Outgoing mail server: smtp.office365.com

      iv) User name: Enter the email address of the mailbox with the permission to "send as" the shared mailbox.

      v) Password: Enter the password of the mailbox with the permission to access the shared mailbox (same as step 'iv' above)

      vi) Tap "Next".
  11. Tap "Save".
    iOS Shared Office 365 Mailbox Step 5
  12. Switch off the "Notes" sync which should leave just "Mail" switched on.
  13. Tap "Save".
  14. Exit the Mail app by ending the task (double press the Home button and slide the Mail app off the top of the screen).
  15. Enter the Mail app and check that the account works.


Source.