How to tackle FPs with RCVD_IN_*

[Ramprasad]

Hi,
 I am using SA 3.1.0 ( + many SARE rulesetes ) for my Antispam cluster
of machines.  We get a huge traffic and by and large the solution works
fine. Only problem is when legitimate senders use dialups etc. Their
source IPs get listed in a lot of BL's and in effect their mails get
marked spam.
  
  I assume , this would not be a unique problem to my case. What do
others use to tackle these FP's. I have already reduced scores of some
BL's but now I risk letting some spams thru. 

THanks
Ram