Allow & Block List
Once logged in, click on the "Allow & Block List" link on the left side of the screen
Type any email address or domain name into the "Add Blocked Email Addresses & Domains:" field on the far right side of the screen*
Click "Add" at the bottom of the screen
* Only add domain names of trusted companies. Try to avoid adding general domain names like gmail.com, yahoo.com, hotmail.com, etc.