Safe List of Senders in Windows

Please clarify this for me. I may have an email in my 'junk' folder.
When I click on 'Mark as Safe' a notification in yellow appears:
" Your list of safe senders is full. To add more, remove addresses until there are fewer than 250 safe senders. "

How do I comply with this instruction?

Anther one: When I do not want to get email from a site I copy and paste its
url in the 'block sender' box.
Incidently, it was an advise given me by SnM.
In some cases am told: " This cannot be performed as the sender is in your 'Safe List'."

How do I access this 'safe list' to eleminate the offensive sender?

Thank you very much,
ben