Smart Screen® email filters are influenced by a number of factors related to the sending IP, domain, authentication, list accuracy, complaint rates, content and more.
As a result, emails from new IPs are more likely to experience deliverability issues.
Microsoft does not allow this behavior and takes action on IPs that engage in it.
If any of your sending IPs is blocked for namespace mining, please check that your machines or email sending accounts are not compromised by an attacker who may be using your servers to harvest email addresses, and ensure that any method you use to validate email addresses does not use namespace mining techniques.
These services have been tailored for senders and for ISPs.
To learn more about the Sender and ISP Services, go here.