Definition: A whitelist is a database of email and IP addresses deemed "safe." Incoming emails from these senders is allowed and not blocked by a blacklist filter.
Email service providers maintain their own whitelists and blacklists of domains and senders. If a service detects a large number of spam emails coming from one particular domain, then the host will shunt emails from that domain directly to spam from then on. Alternately, providers maintain whitelists for trusted domains which will always appear in a user's inbox, whether the email is solicited or not. For the major providers, there is no way to request placement on these lists - inclusion is based solely on a domain's reputation.
Individual users also have their own whitelists, which supersede an email provider's blacklist. Users can add domains or individual email addresses to their whitelist manually, although most do not. Instead, user whitelists grow via the platform's automated systems. When a user replies to an unsolicited email, for instance, some systems will automatically add that domain to the user's whitelist.
Email providers use several metrics to determine a sender's reputation: previous spam complaints, subscriber activity, whether recipients actually open the emails or just delete them, and email content including indicators such as a text-only message, one that carries no images or attachments.
As an email marketer, it's important to know how to get on trusted senders lists and avoid looking like spam: