Configuring preferences and options in Outlook.com
Hi. I'm Brian.
In addition to what the AI states, you can, instead, make sure the address is one of your Contacts. Contacts are always considered safe senders. Also, keep in mind that there is a limit to the number of entries in the safe sender list. If you have a domain from which you often receive messages (multiple senders all using that domain), then add the domain itself to the safe sender list rather than each individual sending address.