Automated IP blocklisting mechanisms like CSS (and XBL) have a zillion rules and are (IMHO) perhaps occasionally susceptible to the occasional false positive from an overzealous rule. This might stem from something harmless but real (how you HELO when executing email testing from a host, maybe using something like Swaks) or a ghost indicator (faked header that shouldn't have been trusted), or who knows what.
We're never going to know what happened exactly, unless somebody contacts Spamhaus and they choose to provide info. (Or if something went so badly and broadly wrong that enough members of the peanut gallery could figure out what's up by comparing notes. Doesn't seem to be the case here.) So after you've reasonably confirmed that there's no SMTP funniness running on that server, no unexpected NAT traffic allowed out on port 25, etc. etc. etc. the next step is going to be to reach out to Spamhaus to ask them to review and see if they could confirm or disprove the theory that it's a bug-related FP. Cheers, Al Iverson -- Al Iverson // 312-725-0130 // Chicago http://www.spamresource.com // Deliverability http://www.aliverson.com // All about me https://xnnd.com/calendar // Book my calendar
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