https://github.com/HotCakeX/MicrosoftDomains
...and microsoftonline.com is not among them (unlike microsoftonline.net and other variants). But it seems to have been registered in 2002, and the record looks legit:
It's definitely a Microsoft owned domain and actively used - for example in Azure Active Directory (Entra).
I did not expect 645 entries!!
That is insane.
but microsoftgenuinerewardsrc.com is! shameful!