14 DAY TRIAL // Blacklisting is one good security measure that many IT managers decide to employ. Basically it's a list of e-mail addresses of known spammers and this acts almost like a filter. It may also contain websites that are considered off limits or dangerous because they may be part of frauds or because they are malware infected. It works in a simple way - admins set it up and then allow access to everything on the web except for anything from the blacklist. Also, besides basic spam guards, users that have a blacklist will never receive any message from anyone on it. It's like the basic "ignore" function all instant messaging services have.
So, what is so complicated about the blacklist? You get spam from someone - you put them on the list... well, you could do that, or you could add them before they spam you, so that you will never receive any unwanted message. Most people are going to ask: "So, how do I do that and is it possible to ignore innocent users, by mistake?" Well, the answer is simple - there are a lot of pre-made blacklists that you can find on the web. They contain the addresses of world renowned spammers. You can add those to your list without any doubt. But be careful, the longer the list, the higher the chance you might ignore "clean" users.
If I had my own business and I were making a blacklist I'd put on it the IPs from my area that I know for sure to be spammers and everyone else that I wouldn't need to contact. I'm only working with Romanians, only inside the country and never export anything, but I would put all the IPs from outside Romania on my blacklist. Why allow them to send me any messages since I wouldn't care about them anyway?
So, yes, blacklisting is efficient, but you can ignore some people by accident, but there is no perfect way of spam filtering yet...