When sending Secret information with a letter of transmittal, what special markings must be applied?

When sending Secret information with a letter of transmittal, it is essential to include a notice that states "the letter is Secret when separated from classified enclosures." This marking serves to indicate that while the letter itself may contain unclassified content, it is still associated with a classified matter. Therefore, the letter's classification level must be communicated clearly to ensure that individuals handling the document understand its sensitivity.

This practice prevents unauthorized disclosure and maintains the integrity of the classified information in accordance with proper marking and transmission protocols. It highlights the importance of not only protecting the classified enclosures but also ensuring that accompanying documents are appropriately identified to mitigate risks associated with mishandling or misinterpretation of their content.