When a company's email address or domain name is added to a particular individual's email address book, this prevents unintentional filtering out of emails that the consumer requested to receive.
One of the reasons to whitelist an email account is because you may be receiving too much spam and conventional blacklisting methods are not working.
When enabling whitelisting, you are given three options, Silent mode where emails received from an unknown sender are silently placed into a "rejected" folder, Manual mode where an email will automatically be sent to an unknown sender indicated that permission to send is required and Auto mode which will automatically send the unknown user a link to an address book sign up form.