We use as many as possible, but something is not considered spam or rejected if it is only in one rbl, it only increases the score slightly.

For a very aggressive rbl, use bl.spamcop.net (will mark some legitimate mail as spam though). Some other common ones:

list.dsbl.org
relays.ordb.org
sbl.spamhaus.org
opm.blitzed.org
ipwhois.rfc-ignorant.org

It's really kind of sad the amount of processing power every piece of message needs to go through to combat spam and viruses.

Cheers,

Alex
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Gossamer Threads Inc.