Allow & Block List
Once logged in, click on the "Allow & Block List" link on the left side of the screen. The lists are separated into "Allow" and "Blocked".
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.