For those who have wondered why the feds cared about what former CIA Director David Petraeus was doing on his private email account, recent reports on hacks into the personal computers of former Chairman of the Joint Chiefs of Staff Mike Mullen provide at least a clue. Mullen’s personal computers, which he used while working on the grounds of the US Naval Academy since his retirement last year, were subject to attacks that the FBI reportedly traced to China. The hackers also gained access to Mullen’s personal email account. Government officials say this hack is part of a broad campaign of cyber espionage against former senior government officials.

Even if former officials like Mullen don’t have access to classified information, foreign intelligence agencies could be interested in a wide range of information. The former official could be consulting for the government; could have private email conversations with current government officials about sensitive topics, or recount such conversations in his emails with other people; or could be up to no good (say, having an affair), which makes him potentially vulnerable to blackmail. Information gleaned from a former official’s private email thus could be a treasure trove for foreign spies. The only limiting factor is resources—how many hackers and analysts do foreign intelligence agencies have to process such information, given all their other targets.

By comparison, the private emails of a current senior government official—let alone the CIA Director—could be exponentially more valuable. People may differ about whether Petraeus should have resigned from office over his affair, but there’s no serious question that there was legitimate counterintelligence interest in determining whether Petraeus’ private email was being hacked and whether Petraeus was doing something that might make him vulnerable to blackmail by a foreign intelligence service.