Managing personal Outlook.com account settings, security, and privacy
I get these too, however the gmail addresses can be somewhat cosmetic and spoofed despite their apparently good spam filtering.
When you send the report to MS, it puts the header information in the email text. The sender IP address can reveal the domain of the host ISP if they are not routing through VPN or other obfuscation. Ive found reporting rogue IP to Amazon AWS to be very responsive and cooperative in shutting down offending IP addresses, not so with Google domain services which doesn't seem to care.