Yes, you can configure FortiADC to authenticate OWA users by using the client authentication method as the HTML Form.
I have shared a sample configuration. In this example, I am using the LAB Network Environment mentioned below. Unfortunately, I don't have an Exchange server in the lab, but the configuration steps are similar.
Add an LDAP server Entry
Add a User Group: Select the Client Authentication Method as ‘HTML form’ and Member as the LDAP server entry (added in the previous step)
Create an Authentication Policy and select the User Group added in the previous step.
Call the Authentication Policy in the Virtual Server.
Access the website. FADC presents the HTML form to the client.
If the user credentials are correct, fortiADC forward the request to the webserver.
You can configure Authentication Relay (User Authentication>Authentication Relay) with Delegation Type set to 'Kerberos" and then call the Authentication Relay in the User Authentication>User Group (used in the Authentication Policy)
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.