To sum this up, I have found that there are ADMINISTRATIVE rules and regulations within the federal agencies and departments that prohibit sending classified information over unsecure channels deal with the markings on classified documents.
But I haven’t (yet) found any federal laws that apply.
In summary:
1. The markings DO NOT matter.
2. The markings do not make the info in the document classified.
3. The markings are just a safety precaution.
4. Only the contents matter.
People run afoul of administrative rules and regulation all the time, such as EPA regulations for example.