I can't give you a number clearly, but if someone received that email it means that flight was adjudicated to be outside the carrier's control, whether that be due to weather or an airport facility or whether that be that an airplane could not operate for reasons prescribed for safety under the regulation.
Again, without knowing the specifics of the event, I can't comment, but clearly that flight's been adjudicated to be outside the carrier's control as prescribed by the regulations.