If you have a dynamic IP from your ISP, could be you got unlucky and were given a address previously used by attackers.

Or if you have a static IP on a VPS or similar, they may have had a lot of attacks from the IP Range.

By attacks in this instance I mean people setting up phishing or similar websites as the most common example. A simple web form, probably with obfuscated code. They then send a bunch of emails line "click here to view your invoice"and gather office 365 credentials.

While it's not good that this kind of false positive happens from time to time, I am more thankful this kind of service exists. Yes, there's privacy and security implications, but smart screen has stopped legitimate attacks at our clients before, and we force it enabled wherever possible.