Today I noted a ticket in which a client of ours believed himself to be blacklisted due to results from

http://mxtoolbox.com/blacklists.aspx

When he searched for his IP the following appeared:

Blacklist Name: xxxxxxxxxxx
Status: LISTED
Reason: blocked, See: Detail
Return codes were: 127.0.0.1

Note the return code is the loopback IP - that's the key here. That means that ANY IP you run a check on will show up as 'listed' for this particular blacklist. I've seen this happen a few times in the past when a blacklisting service shuts down (usually due to being DOS'd or so forth). They set the return code on their list to always return the loopback, which shows every IP as listed and as such messes with anti-spam measures since it continually gives false positives. Should you ever see this ignore it, but make sure your own anti-spam measures no longer use the bad blacklist.