Allow & Deny List
Once logged in, click on the Allow & Deny List link on the left side of the screen. The lists are separated into Allow and Deny.
Type any email address or domain name into the Sender e-mail, domain, or IP address field on the far right side of the screen (towards the top)*
Click Add at the bottom of the interface.
* Only add domain names of trusted companies. Try to avoid adding general domain names like gmail.com, yahoo.com, hotmail.com, etc.