Whitelist

"A whitelist is the opposite of a blacklist. Instead of listing IP addresses to block, a whitelist includes IP addresses that have been approved to deliver e-mail despite blocking measures. It is common practice for ISPs to maintain both a blacklist and a whitelist. When e-mail service providers, like Constant Contact, say they are “whitelisted” it means that their IP addresses are on a specific ISP’s whitelist and are confident that e-mails sent using their service will be delivered."
Marketing Strategy: Spam-Free e-mail Marketing to Customers