How to unblock a sender in Outlook:
In the Outlook menu bar select Tools, then Junk Email Preferences.
If Junk Email Preferences is greyed-out, your email account may be hosted by Exchange.
On the Blocked Senders tab, select the email address or domain you wish to unblock and choose Remove the selected sender button.
Users can also unblock a sender on Exchange based accounts, such as work email accounts hosted by Exchange.
For those accounts, junk mail filtering is handled on the Exchange server and is not a client option.