Open relaying is where all the problems start. It is actually the state of a server sitting out on the Internet to which anybody has access. Email servers that are configured to run wide-open are a major access point for spammers. As soon as an SMTP server is placed online, if it is not configured properly, it is hijacked. Back in the olden days, the default option was to allow anybody and everybody to send through a mail server. Eventually the default for new operating systems was to keep the relaying closed. Some SMTP servers remove the open relaying altogether to prevent accidentally making it open. With open relaying, the server is guaranteed to be used by someone else to send spam messages and cause the server to get blacklisted. Many mail agents check the server settings by sending a test email to see if the SMTP accepts it. If the test is accepted the recipient server may decide not to accept the email from that server, legitimate or not. Never, ever use open relaying.
One of the traditional methods spammers use is finding an SMTP server online without any restrictions in place and use it to send their emails. This can effectively hide their tracks and could cause potential problems for the legitimate server they confiscate. Several online organizations have taken it upon themselves to identify potential spammers and stop them from using other peoples open mail servers to send spam. The IP address of the server that appears to be open, is added to a database that is checked by subscribers and set to deny emails automatically. If this happens to a legitimate business, contact the database administrator to request that your company be removed from the list on the condition that the problem with the server be corrected.
Closed relaying, like open relaying, is merely a description of the state of the server and is not a setting found in the relaying options. Closed relaying means that some sort of authentication must occur before someone can send emails through the SMTP server. The best way is to list a computer's IP address, or a block of addresses can be added to the server in the relay settings section, which vary by product. Enabling these features restricts access to the SMTP server and thereby closes it to people who are not approved. Further restrictions include authenticating with a username and password, and blocking connection requests from all but an approved list of computers. Most mail servers are now closed to all traffic, until a computer is specifically added to the list, or can authenticate.