Outlook Web App

Safe senders are people and domains you want to receive email messages from. A safe recipient can be an individual mailbox or it can be a group. For example, you might want to allow messages that are sent not only to you but also to a particular person. 

1. Click the Settings button in the top right corner. Then, click Mail under Your app settings. 
2. Under accounts, click Block or allow.
3. In the Safe senders and recipients box, type an email address or domain, and then click the add button (+).
4. Click Save at the top of the screen.