You'll get it when Microsoft decides you are a spammer for no other reason then sending email from port 25 from your house. Or when you can't seem to sign up for a service... until you use your old Gmail address.
Yeah, there was (is?) a period of time where viruses were used to send spam so if you got infected you'd suddenly be sending out a lot of SMTP traffic from a residential IP address. The entire industry adopted the practice of not trusting residential ips. Then the spammers shifted to cheap VPS providers and ip and netblock black lists became more common.
