If you go to settings on the left bottom and click on it
The second main option will be filtering
Under the filtering option you will have content filtering, and when you click on the content filter
you will see a content filter rule named "Internal Blocked Senders". Edit that rule and on the second page you will see a text box with a list of currently blocked addresses, you can add or remove items there.
Note : The rule name may vary depending on the name given while creating it.