To whitelist an email address just means you add them to your approved senders list. This tells your email client that you know this sender and trust them, which will keep emails from this contact at the top of your inbox and out of the junk folder. There are a few ways you can do this.
Add the address to your contacts:
Add the email address that is sending you email (for example: firstname.lastname@example.org) to your Contact List. Most email providers will usually deliver email from addresses that are in your Contacts List.
If you find our emails in your “Promotions” Tab (Gmail users):
Go to your Promotions tab and locate an email from us there
Simply drag this email over to the Primary tab of your inbox
Yahoo Mail Users:
When you open the email message, you should see a “+” symbol directly next to the From: and the Sender’s Name. Click on the “+” symbol and then Add To Contacts. A pop-up should appear. Select Save.
Outlook 2003 & Later Users:
In the list view of your emails, locate an email from us. Right click on the email message in your inbox. Then select Junk E-mail from the contextual menu. The click on Add Send to Safe Senders List in that submenu.