My experience is that Outlook will accept email from a new IP ... for awhile. But rather quickly they block it. I think they watch for a commercial amount of traffic, enough to get a reputation from various services. Not enough traffic for a reputation? Very suspicious, adios... How else would legit commercial email senders bring up a new server?

From the MailOp commercial mailers mailing list, it seems Outlook will eventually unblock you, but you have to keep appealing, etc., for multiple rounds.