I am not a GT Server user, however, I thought I'd post some other blacklist servers that may be helpful for others configuring Spam Assassin or other Anti-Spam server or client products....

relays.visi.com
blacklist.spambag.org
hil.habeas.com
ztl.dorkslayers.com
blackholes.intersil.net
blackholes.wirehub.net
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Buh Bye!

Cheers,
Me