After doing research on the bl servers I settled on this list below.  Any time I have used spews lists I get too many false positives.  I am now using minimum two maps servers before blocking.-- using one was too agressive for our use as an ISP.

blackholes.five-ten-sg.com
bl.spamcop.net, true
blackholes.mail-abuse.org, true
dialups.mail-abuse.org, true
relays.mail-abuse.org, true
spam.dnsrbl.net, true
dnsbl.njabl.org, true
blackholes.wirehub.net, true
relays.ordb.org, true

That's a pretty good list.  You're absolutely right about using minimum two servers... some of them can gen quite a few false positives.  Spamcop is great, definitely not to be used in a one-list-only scenario, but spamcop+confirmation by someone else is proof positive of spam.  Considered ordb... they are good at testing/listing open relays, but I've avoided them due to a reputation of being very slow to retest and remove once a mail admin has cleaned things up.

Spamhaus is another list I've used with great results. Their scope is very limited compared to almost any other list -- they focus their efforts completely on the large-scale, repeat spamming operations.  This means that they don't block your ISP-hopping, spam-and-dash types, but conversely it also means that they almost NEVER generate a false positive -- if they say it's spam, there's a 99.999% chance that it is.

sbl.spamhaus.org, true

So you feel that Spamhaus and Osirusoft are both too aggressive at blacklisting?

No, as a matter of fact I use sbl.spamhaus.org now.  I do find spews blocking a bit too broad for my use. I know that spews is trying to do a good thing but blocking of entire isp's and class-c space does not work for us.  What is the relationship between Osirusoft and spews if any??

Dwight,

Can you confirm that blackholes.five-ten-sg.com does not requre the ",true" after it as the other DNSBL's do?

I am using a true.  Let me know if you find otherwise.

Thanks