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Outlook on Web
Click the setting top right corner (the picture of a gear) 
Select "View All Outlook setting" located at bottom of the settings window
Select Mail
Select Junk Email
You will see your blocked and safe senders list.
To add a user to the block senders and domains section or to the safe senders and domains section, click on the +Add button, type in the email address and press the enter key.
To remove a user to the block senders and domains section or to the safe senders and domain section, select the email address and click on the trash can.
Outlook Client
From with the Outlook client, under Home, click on the the Junk icon, and select Junk Email Options
Within the Junk Email Options window, there will be various tabs, select the tab in which you want to review for either safe senders, safe recipients or blocked senders. With each tab, you can add, edit or remove an email address to by clicking on the add, edit or remove button. Once added, removed or edit, click the ok button to go back to the Outlook client inbox.
If this is a shared mailbox, please visit Article - Email: Junk Email Settings ... (teamdynamix.com)