Sign in to your account at Yahoo! Mail and do the following:

Add the address to your contacts

Add the email address that is sending you an email (eg to your Yahoo! Mail Contacts by clicking on the contacts icon:

image of the yahoo UI and where to add an address

And clicking Add a new contact:

Yahoo UI where to add a new contact

Create a filter for the address

  • Mouse over the Settings menu icon and select Settings
  • Click Filters and then Add
  • Enter a filter name
  • Enter the domain of the email you want to whitelist in the Sender field, or
  • Select Inbox as the folder to deliver the email to
  • Click Save and then click Save again