[Exim] RBL/anti-spam advice?

Up until a few months ago,

rbl_domains =
proxies.blackholes.wirehub.net/warn:proxies.relays.monkeys.com/warn:relays.ordb.org/warn:bl.spamcop.net/warn:dnsbl.njabl.org/warn
# check all hosts other than those on internal network
rbl_hosts = !^(.*\.|)(DOMAINS)$

[ in exim 3 config ] along with a $header_X-RBL-Warning: test in a
filter would get rid of nearly all spam, but now *a lot* is let through
to the main mailbox. The mechanism still works in the sense much spam is
detected via this rbl_domain setting, but the proportion that isn't has
increased dramatically. Any ideas why? Have the spammers got smarter
lately, or has something happended to the above mentioned lists? Can
anyone recommend other RBL hosts? Should I perhaps use SpamAssasin or
similar?

- Toralf