Today after I posted links to our new Spam Filtering Troubleshooting Tools, I received an email raising concern about our use of the controversial SPEWS blacklist.  Here was my response:

> I agree with you. SPEWS is a very unreliable RBL to use to block mail.
> We don’t use in SPEWS that way. We only use it as part of the
> weighting system. It takes 6 points for an email to be tagged as spam
> (or 8 points if you set your filter level to low). The SPEWS scores are
> very low in comparison to the rest of the RBLs that we use. There are
> two SPEWS lists and we use them as follows…
>
> RCVD_IN_SPEWS1 Received via a relay in l1.spews.dnsbl.sorbs.net 0.701
> RCVD_IN_SPEWS2 Received via a relay in l2.spews.dnsbl.sorbs.net 0.301
>
> SPEWS works very well as part of a weighting system. It is a good
> indicator of exactly what it says it’s purpose is for – “spam early
> warning”.

A great feature of our system is that we never reject SMTP connections based solely on any single RBL.  IPs must be listed in multiple RBLs or have additional spam checks fail in before we will reject the SMTP connection.  And the SPEWS RBL is not part of that equation.