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Effectiveness

While there are many ways to measure the utility of an anti-spam solution, the bottom line is high spam-blocking effectiveness. Effectiveness is the percentage of spam messages correctly filtered as spam.

Maintaining high effectiveness is challenging for mainly two reasons. First, the "spam" that constitutes the final few percentage points can often be quite subjective, different people have different opinion about spam. Therefore, apart from a universal spam filtering mechanism, tools should be provided to users to fine-tune for best spam-blocking results. Next, an over-aggressive drive towards effectiveness increases the chances of false positive (legitimate mail being mistakenly blocked).

Accuracy

Another important criteria for an anti-spam software is accuracy. Accuracy refers to the ability of a spam blocker to distinguish legitimate messages from spam. The more legitimate mail being blocked, the higher false positive rate it will cause.
Email spam Filters which are over-aggressive will usually result in high false positive rate. As false positive increase, users are forced to manually wade through their quarantines to pick us mails from spam, a very time-consuming and tedious work. False positive is what a spam filter shall avoid by all means as it is often more anti-productive than spam itself.

Minimal Administration

Many spam blockers require substantial training for initial and ongoing anti-spam effectiveness. This will cause a lot of administrative work. A good spam filter should be something you can just install and forget, it will do the filtering just as you will require. Minimal administration work is another important measure of a good anti-spam solution. SpamWeed also directly integrates with Outlook and Outlook Express, which further reduces users' administration work.