there are two dns zones. relays.nether.net and unsure.nether.net.
relays.nether.net lists hosts that are confirmed open-relays
unsure.nether.net lists hosts that can not be tested
trusted.nether.net lists hosts that are trusted to not be a spam source

usage requires one of a few things:
access to your own sendmail.cf or
access to your SpamAssassin config

the dns zones are structured in the same way as all the other dns-based open relay systems. I recommend checking the MAPS(sm) website for more information.

We return 127.0.0.2 for hosts that are listed in the database.

Example SpamAssassin Usage:

header X_UNSURE_NETHER          rbleval:check_rbl('set-firsthop', 'unsure.nether.net.')
describe X_UNSURE_NETHER        DNSBL: sender is possibly open relay
score X_UNSURE_NETHER 0.2

header X_RELAYS_NETHER          rbleval:check_rbl('set-firsthop', 'relays.nether.net.')
describe X_RELAYS_NETHER        DNSBL: sender is possibly open relay
score X_RELAYS_NETHER 2.0

header X_TRUSTED_NETHER          rbleval:check_rbl('set-firsthop', 'trusted.nether.net.')
describe X_TRUSTED_NETHER        DNSBL: sender is good mail source
score X_TRUSTED_NETHER -5.0