We are a small ISP, today we had the very same problem, go to this link http://go.microsoft.com/fwlink/?LinkID=614866 (from their FAQ https://postmaster.live.com/snds/FAQ.aspx#FeedbackAndQuestions) and complete the request form
they told us we have been "conditionally mitigated" (unblacklisted) and finally our emails to their accounts gets delivered, still marked as "junk": we are still not able to know why our mailserver IP has been blacklisted, we are still waiting for a complete reply, if we'll evet get it
ciao Giovanni