Short answer: Personal Microsoft addresses (outlook.com, hotmail.com, live.com, msn.com, and their many country variants) come back as unknown, not invalid, for the same reason as iCloud: Microsoft's consumer mail system does not reveal mailbox existence during verification. These results are not charged.
Why this happens
Microsoft's consumer mail platform actively defeats mailbox-level verification. Their servers do not give a trustworthy per-mailbox answer during the SMTP conversation — probing returns inconsistent or misleading responses, and high probe volume gets rate-limited or blocked at the network level.
Rather than guess, we intentionally skip live probing for these consumer domains and return an honest unknown. This matches how the major verification services handle the same domains — the limitation is on Microsoft's side.
Note: business Microsoft 365 domains are different
If your list contains custom company domains hosted on Microsoft 365 (for example name@yourcompany.com where the company uses Microsoft), those are verified live. However, many Microsoft 365 tenants behave as catch-all (they accept mail to any address), so those often return catch-all rather than a clean valid. See Why is an address marked catch-all? for what that means.
Is there a configuration to fix it?
No. There is no setting that will turn personal Outlook/Hotmail/Live results into valid. We prefer an honest unknown over a fabricated valid.
What we recommend
Treat unknown personal-Microsoft addresses as "unconfirmed but likely fine to send." If they came from a legitimate source, they are usually deliverable. Do not discard them on the assumption they are invalid.