Taken from the OE help files:-
"To manage the blocked senders list
On the Tools menu, point to Message Rules, and then click Block Senders List.
To remove a sender or domain from the list, select the name and click Remove.
To change a sender's name or domain on the list, select the name and click Modify. Make your changes in the dialog box and click OK.
To add a new sender or domain to the list, click Add. Enter the information in the dialog box and click OK.
Click OK in the Message Rules dialog box to apply rules that have been added or changed."