Top secret /special access programs material is highly sensitive info, whether marked as such or not.
The “ —unauthorized disclosure— could cause exceptionally grave damage to national security”.
The release of the —information—, marked accordingly or not.
Here’s the relevant guidelines from the Dept of the Navy:
http://navyadvancement.tpub.com/12018/css/Security-Classification-Levels-693.htm
Top Secret Top Secret is the classification level applied to information whose unauthorized disclosure could reasonably be expected to cause exceptionally grave damage to the national security.
Some examples of information that could cause grave damage to national security include—
Armed hostilities against the United States or its allies
A disruption of foreign relations vitally affecting the national security
The compromise of vital national defense plans
The disclosure of complex cryptographic and communications intelligence systems
The disclosure of sensitive intelligence operations
The disclosure of significant scientific or technological developments vital to national security
All true, but the article says the emails were "clearly marked" when they weren't.