Below is the template on how to redirect Mac users to a different realm. You can use this for Office 365 or other realms if they're configured for IWA:
1. Open IIS
2. Go to Sites > Default Web Sit
3. Select the realm that you want the redirect to occur at.
4. Open URL Rewrite.
5. Create a blank inbound rule.
6. Click "Add" in the Conditions section and follow the rules below.
7. Set the redirect path to another SecureAuth realm.