A white-list is a list of email senders whom you trust and would like to receive mail from. Conversely, a blacklist contains those that are not to be trusted. Blacklists need to be configured and administered on a server, at the ISP (Internet Service Provider) level or on your network. If you do not have such access, and most people do not, you can use the blacklists by choosing email services from companies that use such blacklisting techniques.

A mail server is designated as having an open relay when mail is processed in a location that is not local to either the sender or receiver. The mail server is unrelated to either party, and as such, has no business processing that email. Think of driving from your home in Washington, DC to your friend's home in Boston. If you pass through New York City on the way, that is to be expected. However, if you stop in Dallas, the route becomes suspect. An open relay mail server, whether intentional or not, is allowing mail to be routed through it that shouldn't be.