We noticed problems on the external Blacklist Service "Burnt Tech" this weeekend around 1am on Sunday Sept 2015. Our monitoring service alerted us to say that any IP addresses we hold were infact blacklisted.
This was a false-positive and returned to an "unlisted" status around 7am. Investigation shows that the website is currently pointing at Sedo parking which suggests the service is no longer operational and the domain is for sale.
If you experience any alerts from services such as MXToolbox.com over the weekend, it was probably related to this. We suspect MXToolbox.com will remove the service if it does not return to a working state.
Update: As suspected, MXToolbox has removed the Burnt-Tech blacklist from their monitoring service, more info.