If you've come across the domain fa000000113.resources.office.net.mcas.ms and wondered whether it's legitimate, here's the short answer: it is. This subdomain sits under Microsoft's own network and is almost certainly part of the Microsoft Cloud App Security (MCAS) infrastructure. The SSL certificate is issued by Microsoft, the site enforces HTTPS, and there are no blacklist or malware flags.
For a backend resource URL, this level of transparency is more than adequate. You'll find privacy and terms pages, along with a business disclosure. While there's no direct contact form on the homepage, that's normal for an infrastructure endpoint that isn't meant for direct consumer interaction. If you were redirected here by a Microsoft service or application, there's no reason to doubt its safety. Just keep in mind that this isn't a consumer-facing website — it's a piece of Microsoft's technical backbone.