CiviCRM does not send mail to microsoft based webmail clients (outlook, hotmail and live)
Microsoft's mail services have blocked your mail server.

Their specific reasons for blocking may vary (and their server's response codes will guide you here to some degree), but there are a variety of things you can do to improve the situation.


Check your mailserver's reverse DNS matches your domain
Check the domain in question's SPF configuration permits your server
Ideally, configure DKIM for outbound email on your server
Join Microsoft's Junk Mail Reporting Program (JMRP) and "claim" the IP address of your mailserver
If JMRP reports your server blocked, submit a request to unblock your server


Alternatively you might consider one of the hosted mail provider plugins available for CiviCRM - managing relations with mail providers can be a big task, and outsourcing that may save you a lot of time.