I guess you know the ordb.org blacklisting service for mails – but did you know that it was taken offline long time ago? It did not report spammers since that time, but nevertheless many mailservers are still querying it – so the operators were forced to do something against that.
Since yesterday the rbl service answers ‘listed’ to any query, no matter if it is a mailserver sending spam or not. So if your mailserver is using this rbl, it will reject any messages. Rough education.