The Norton DNS is a great service, but the 'typo' correction feature that returns an IP address instead of a non-existant domain (NX-DOMAIN) produces a little fun for folks who utilize URI blocklists.
A few other DNS providers have had similar issues
- surbl.org
- uribl.com
- uribl.org
- spameatingmonkey.net
- dob.sibl.support-intelligence.net
I'm sure there are others, but those are the majors. Returning any IP address, even if it's not in the loopback range will have most antispam software treat it as a positive result.
Thanks!