More than a month ago, we have received an alert from Blacklist Monitor that our main server became blacklisted on Spamhaus SBL blacklist. This was a problem for us because we do have our own mail server that sends emails to our users and suddenly, our emails were silently blocked in many systems because of this listing. And we started our investigation. Why were we blacklisted if we did not send any spam?
The monitor told us that our listing details can be found on http://www.spamhaus.org/sbl/query/SBL245859, so this was the first information to read. The information on that page was very clear: