Using the OLD Outlook
To add people to your Safe Senders List, do the following:
1. On the Home tab in Outlook, in the Delete group click Junk, and then click Junk E-mail Options.
2. On the Safe Senders tab, check the Automatically add people I e-mail to the Safe Senders List box.
Using the NEW Outlook
1. Go to Settings (gear icon) → Mail → Junk email.
2. Scroll down to the Senders section, then find the Safe senders and domains tab.
3. Click + Add safe sender.
4. Type the email address (or domain) you want to mark as safe.
5. Click OK / Add (depending on version), and then Save your settings.
If you check the Also trust e-mail from my Contacts option, your addresses in Contacts are considered safe senders.
1. Click the Mail icon, and then click the Home tab.
2. In the Delete group, click Junk, and then click Junk Email Options.
3. On the Safe Senders tab, clear the Also trust email from my Contacts check box.