To provide spoofing protection to users you can import from Office 365. 

You can import all users or just users in specific groups. Only users that have been imported will have spoofing protection enabled. 

Company Settings -> Email Addresses

Click on the Office Icon

The importing process is designed to sync users every day. If you add or delete users from your directory they will be modified in the system. 

Sign in to Office 365 with an Admin account

Select the option to stay signed in 

Once an Admin account is authenticated you will see the option to import users. The default is to Sync all users.

You can optionally import users from selected Groups

Select the group to import users 

The users will be imported into the portal