Step 1: Login to outlook webmail
Step 2: Click the setting icon in the top-right corner and then More mail settings
Step 3: Select Safe and blocked senders and then Safe senders
Step 4: Add add the domain of the email you want to whitelist to the list of Safe senders
Step 5: Return to Safe and blocked senders and then select Safe mailing lists
Step 6: Add the email address of the online group you want to whitelist to the list of Safe mailing lists