See below the guide to add a shared mailbox calendar to the (native) mail app on the phone.
- Go to Settings > Mail, Contacts and Calendars > Add Account.
- From the choices available, touch Other near the bottom.
- On the Other screen, touch Add Mail Account.
- In the new screen, you will enter:
- Name: The name that will be displayed to recipients on email.
- Email: The email address of the shared mailbox (e.g., email@example.com).
- Password: Your Tekup email password.
- Description: Your chosen description.
- Touch Next to get to the full settings.
- On the following screen, confirm that IMAP is selected at the top.
- Scroll down. In the Incoming Mail Server section, enter the following values:
- Scroll down. In the Outgoing Mail Server section, enter the following values:
- Host Name: smtp.office365.com
- User Name: Your full tekup email address.
- Password: Your tekup email password.
- Touch the Next button and wait for the server to verify your settings.
- Choose your synchronization choices and touch Save in the upper right corner. You will now find the shared account in your Mail app under Mailboxes.
I hope this helps, any further questions do raise a new support ticket.
Office 365 Support