Graymail is the practice of discouraging a prosecution from proceeding by contending that a defendant may need to disclose classified or sensitive information as part of a full defence Such an approach can force the government to choose between dropping the prosecution or allowing the information to be disclosed at a trial
Before 1980, some officials in the USA escaped prosecution by threatening to disclose unspecified secrets in open court. Congress enacted the Classified Information Procedures Act in 1980 to ensure that the government was not surprised by any disclosures at trial. If the defence intends to use classified information, it has to inform the government, and then the two sides argue before a judge in secret on whether the information is needed for full defence. If a judge decides that the defendant is entitled to the information, the government has to decide whether to accept the likelihood that the information may be disclosed in a trial or drop the prosecution.