I made a discovery today that surprised even myself. Using the rescore masscheck and weekly masscheck logs while working on Bug #6247 I found some interesting details that throws a wrench into this lively debate.

It turns out that the ReturnPath and DNSWL whitelists have a statistically insignificant impact on spamassassin's ability to determine ham vs. spam. Meanwhile, both whitelists have high levels of accuracy.

How can both of these statements be true? I suspect this is because the scores are balanced by the rescoring algorithm to be "safe" in the majority case where no whitelist rule has triggered. Thus whitelists are not needed or relied upon to prevent false positive classification.

While whitelists are not directly effective (statistically, when averaged across a large corpus), whitelists are powerful tools in indirect ways including:

* Pushing the score beyond the auto-learn threshold for things like Bayes to function without manual intervention. * The albeit controversial method where some automated spam trap blacklists use whitelists to help determine if they really should list an IP address.

spamassassin-3.3.0 has reduced the score impact of these whitelists to more modest levels, maxing out at -5 points. -5 is PLENTY for spamassassin, as 5 points is the level which the scoreset is tuned. Mail from a whitelisted host would need greater than 10 points to be blocked, which is statistically very rare for ham. I believe that we are striking the right balance with these modest whitelist scores in this release.

That being said, whitelists should be constantly policed to maintain their reputation and trust levels. For example, while I currently am impressed by DNSWL's performance, I am not pleased that they seem to lack automated trap-based enforcement. Relying only on manual reports and manual intervention requires too much effort in the long-term for any organization, be it company or volunteer run.

Warren Togami

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