While wondering at high spam scores for NANOG mail, I noticed that it's
in part because of broken reverse DNS for mail.nanog.org's IPv6 address.
The address is 2001:1838:2001:8::20, with reverse delegated to
ns1.scservers.com and ns2.scservers.com... but those hostnames don't
resolve. The auth servers for scservers.com return SERVFAIL.