Whitelisting programs are offered by commercial providers, interest groups and partially the service providers themselves. Email service providers register with a whitelisting provider and will be added to the list once they have been checked. They need to fulfil a number of criteria depending on the provider, in order to prove professionalism. The list will be made available to email providers. As soon as an email is received on the mail server of the provider, the IP/domain will be synchronised with the whitelists. If the sender’s mail server can be found on a list, his emails will receive priority. This means, that emails do not need to pass greylisting and throttling, anymore. As a result, spam filters will also apply lower thresholds or may no longer be necessary.