If you are worried about losing good email add this rule to your ruleset:
header RCVD_IN_HOSTKARMA_W eval:check_rbl_sub('HOSTKARMA-lastexternal',
'127.0.0.1')
describe RCVD_IN_HOSTKARMA_W Sender listed in HOSTKARMA-WHITE
tflags RCVD_IN_HOSTKARMA_W net nice
score RCVD_IN_HOSTKARMA_W -5
This is a white list and it is very extensive. It will protect a lot of
your ham while blessing very little spam. So if you are ham oriented,
not losing good email, this is a list that will work for you.
