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.
Servers need to be set up to prevent this unauthorized use. The Mail Abuse Prevention System (MAPS) provides excellent resources and links to assist you in closing and/or upgrading your server to prevent relay-rape. Check out the MAPS Transport Security Initiative pages.