If you run an exchange server you have probably come across mxtoolbox.com - a website that enalbes you to monitor your servers ip and make sure it does not get listed on any email blacklists.
You may have also come across http://www.uceprotect.net/ - which happens to be one of the only blacklist servers that does not allow de-listing as a standard service.
This can be a real pain, especially considering that you can get listed on their level 3 blacklist as a result of your ip block having a high amount of spammers. If you are actually sending out spam, then there is no way to get de-listed (fair enough) but if you are an inocent party, the only way to avoid getting listed in the first place is to whitelist your servers ip address over at http://www.whitelisted.org/ - which provides a paid for whitelisting service.
Makes you wonder how long before Email becomes a paid for only service!!