Unfortunately we find that Microsoft's Outlook/Hotmail service is rather prone to blacklisting senders where the number of received emails from a particular IP is low.
While the internal mechanisms are only explained in very generic terms, essentially Microsoft assigns every IP address a "reputation". This server reputation (or trust) has to be earned over time; where an IP sends little or no email to Microsoft, the default reputation is low and its common to reach this error state.
You can request "delivery mitigation" (i.e. removal from their IP blacklist) here:
You may also like to join their "Smart Network Data Service" (SNDS) program, which can alert you in the future if you are re-listed and sometimes will provide additional information about why the IP has been listed.
To do so just enter your IP here:
https://postmaster.live.com/snds/addnetwork.aspx (Microsoft account required)