Greylisting is a way of reducing the amount of incoming spam and virus emails, in addition to other counter measures. Postgrey is a good choice if you are running postfix.
However, to be able to recieve emails from senders using outlook.com you need to whitelist it. This is because outlook.com is using a different host on each try to deliver the mail, causing delays up to hours, days or the mail not being delivered at all within time limits.
Create/edit the file /etc/postgrey/whitelist_clients.local and add:
service postgrey reload