An email whitelist is a list of IP addresses from which your users expect to receive legitimate mail. When you add an IP address to your email whitelist, mails sent from this IP address will generally not be marked spam. 

NOTE:  If you create a whitelist for your Google Apps account, it will affect your entire domain. 

To add IP addresses to your email whitelist:

1. Sign in to the Google Admin console

2.From the dashboard, go to Apps > Google Apps > Gmail > Advanced settings

3. In the Organizations section, highlight your domain.

4. In the Email whitelist section, enter the IP addresses of your contact's domain host to make sure any mail originating from these IP addresses are not labeled spam. If you would like to add more than one IP address, enter an IP range in CIDR notation or separate each IP address with a comma.

5.Click Save changes.