SpamCop FAQ : Help for abuse-desks and administrators :
While there has been an increase in proxy and formmail exploits for sending spam, open-relay abuse continues to be the biggest hurdle in combating spam. Many of these relays are anonymizing, in the sense they do not accurately record the connecting IP address. Secondly, they do not properly identify themselves, with missing or inaccurate dns/rdns records. Additionally, many are found in netblocks where proper contact and reporting addresses are difficult, if not impossible to find.

In the early days of the Internet, it was considered good citizenship to leave a server available for all to use. Unfortunately, spammers and scammers started taking advantage of these relays to get around limits and terms of their own Internet service providers. It has become necessary to lock networks down tight and deny access to anyone other than authorized users.

In other words, if you leave a mail server open to free use and abuse, you are now considered a bad citizen of the Internet. Hundreds, if not thousands of public and private blocklists and blocking lists have been created with the sole purpose of listing the IP addresses of open relay servers. Users of these lists block all email traffic coming from servers finding themselves on these lists.

Preventing relaying in Microsoft Exchange has a lot of useful and detailed information about a variety of commonly-used weaknesses in one of the most popular email servers.
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