In your hosting control panel (cPanel), under the “Email” section go to “Spam Filters”. In order to ensure mail from a particular email address or domain is always allowed go to the “Additional Configurations” and there you will find “Edit Whitelist”. You can use an asterisk(*) as a wildcard character to allow all users from a domain like in the example here or you can enter specific email addresses:
*@approved-domain.com
Blacklist located within “Show Additional Configurations”