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 assistance Provider) level or on your network. If you do not have such access, and most habitancy do not, you can use the blacklists by selecting email services from fellowships 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 whether the sender or receiver. The mail server is unrelated to whether party, and as such, has no enterprise 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. In this way, spammers distribute large volumes of junk email in order to disguise their actions. Open relays used to be very common, like social mailboxes. You can drop a letter into any one without any consideration of who you are. Spammers ruined this by using open relays to surreptitiously send spam without identifying themselves. When blocked by the administrator or Isp, they plainly move on. They usually scan Ip addresses looking for open relay servers. When they find one, they exploit it.
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Blacklists consist of a database of open relay mail servers and those known to be a source of spam, and are used to filter email coming from those machines. The sender will receive a bounce message or mailer deamon telling them that their email is undeliverable. Often, the sender does not realize that they are on a blacklist or that their server is an open relay. They find out only when they endeavor to send mail to person who is uses a blacklist to filter for spam. If the sender is a spammer, the process is working. If the sender is you, the process is also working because it will force you to take steps to close that open relay or complain to the enterprise who is handling your mail. By windup the holes in the email relay network, the spammers will be forced to stop sourcing their spam in this way. This is but one way to slow down, not stop, the spammers.
Blacklists have a very high probability of false-positives if used in blind faith. Depending on those that contend the blacklist, email sent in good faith could land innocent habitancy and mail servers on a blacklist. This usually happens to one of two ways. The first is through the policy most blacklist providers use to find open relays. They plainly do what the spammers do and scan Ip address to look for a server that will allow them to associate and send mail. Once the schedule finds an open relay they add the server to the blacklist. The ideas administrator of the server is usually never notified. There can be mistakes made in this process leaving the administrator with a huge qoute when mail starts getting lost or bounced back. Second, you may receive an Ip from your victualer that was previously blacklisted, even though it may no longer be an open relay. To avoid this, check the Ip address against the known blacklists and ask your victualer for a new one if it unmistakably appears.
Do you know whether your Isp maintains an open relay? If you discovered that your employee was doing work for person else while on your time, you would unmistakably put a stop to it. Open relay mail servers are doing exactly that and need to be confronted. A mail server should be configured to only relay messages from authenticated users on the network. If properly protected, the server will plainly tell the sender that they do not work for outsiders and bounce the message back. More and more, administrators of servers are blocking the open relay. This has been instumental in slowing down spam.
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