Because Osterman Research is a small company, there are no messaging servers in-house, and we rely on an ISP for mail and Web services. As a result, client-side tools are used for virus and spam filtering.I have used a variety of spam-blocking tools, looking for the best combination of performance, spam-capture efficiency and acceptable false-positive ratio, all with Microsoft Outlook 2002 as the messaging client on a 2.6-GHz Pentium 4 with hyperthreading.While not having conducted a formal review of any of these tools, the three I like best are Qurb, MailFrontier Matador and Alladin Systems SpamCatcher.Qurb is conceptually the simplest of the three tools because it uses a simple whitelist approach to block spam - anyone in your address book or inbox is automatically added to the whitelist when the tool is set up, and adding or deleting a user is very simple. Qurb has a simple preview function in the quarantine, making it easy to determine the content of a message without opening it. Also, Qurb imposes the lowest performance degradation of the spam blockers I\u2019ve tried.On the negative side, Qurb also results in more false positives than either Matador or SpamCatcher because of the nature of its spam-blocking approach - an e-mail message from anyone who has never e-mailed you before is automatically placed into quarantine. Because I receive more than 250 messages each day and often receive messages from new contacts (such as readers of this newsletter), the quarantine has to be inspected much more closely than with the other two systems. I have yet to experience a false positive with SpamCatcher (at least that I have caught) and Matador\u2019s false positive ratio is extremely low as well.All of that said, I would highly recommend any of the three tools mentioned above.However, the choice of a client-side spam-blocking tool depends to a large extent on the type of work you do and the volume of e-mail you receive. If you\u2019re in a business in which you get lots of e-mail and lots of spam, but also many inquiries from people that are contacting you for the first time, a whitelist-based tool might not serve you as well as one that uses more sophisticated filtering technologies based on the message content. If you don\u2019t receive as much e-mail, however, a whitelist approach will work extremely well because you won\u2019t have to spend much time reviewing quarantined content and it will impose the least drain on your computer\u2019s resources.