Simple fix s/1+2=3/2+4=6/ proposed in:
<http://permalink.gmane.org/gmane.ietf.asrg/11079>
Other points:
For lists of domain names the term RHSBL is popular (about
20,000 Google hits), maybe mention this in chapter 3.
In 2.3 replace "127/8" by "127.0.0.0/8 excl. 127.0.0.0/31".
I still prefer to recommed 2.0.0.127 as test entry everywhere,
also for RHSBLs. SURBL uses this among other test entries,
but there's no example.com.multi.surbl.org
The security considerations are incomplete: Combined lists
should never combine completely unrelated sets, especially
not "black" and "white" sets, the users would get it wrong.
Are you sure that you want to discuss a commercial service
like [3] in an RfC ?