how do I find the safe sender list on 365 owa

I am getting error:
Delivery has failed to these recipients or groups:
xxxxxxx@xxx.com
Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept email from certain senders, or another restriction may be preventing delivery.

I want to try to add the email to the safe senders to see if it fixes the problem......other suggestions??