I was looking for something related to this and tripped over this thread.
Just in case anyone else looking to make the password re-use protection feature work properly for Office 365 - as in making Office 365 authentication be the 'trusted' side for the corporate credentials - either synchronised with the on premises AD or stand-alone - it doesn't really matter which, trusted is trusted
The portal.office.com FQDN redirects to the basic 'office.com' so doesn't really play a key part.
The domains that you need to put in to the 'trusted domains section of the 'Zero Phishing Settings' 'protected domains' list are:
login.microsoftonline.com
office.com
Very possibly portal.office.com (I don't believe so but just in case, and it'll do no harm to add it)
Just today I did a step by step test on a new implementation and tried just using login.microsoftonline.com; this was *not* sufficient for the zero phishing to hash the password, even though it redirects to that FQDN prior the the user typing in the login name and password - office.com is essential too!
Funny old thing Microsoft authentication - but add these two to the protected domains and it works a treat! Everyone who sees this for the first time is impressed, just try logging on to any other site with the same credentials after logging in (on a browser) and when the new browser tab opens with the warning about how this is not a secure practice there is always a gasp from the person watching!
Lovely piece of technology, nice work Check Point!