They come as the FBI is probing the extent to which Moscow is carrying out an unprecedented digital campaign to potentially undermine confidence in the political process in the United States.
The nation's top national security officials warned yesterday that the US is prepared to respond to whoever is behind the hacks of political organisations such as the Democratic National Committee.
"Nobody should think that there's a free pass," said Lisa Monaco, President Barack Obama's adviser on homeland security and counterterrorism, when asked at a Centre for Strategic and International Studies conference about the hacks linked to Russia.
A Powell spokeswoman attested to the authenticity of the emails. "We have confirmed that the general has been hacked and that these are his emails," Peggy Cifrino said. "We have no other comment at this time."
Powell's emails were posted on a once-obscure website, DCLeaks, which appeared in mid-2016 to publish material hacked from "top-ranking officials and their influence agents". The site claims it was launched by "American hacktivists who respect and appreciate freedom of speech, human rights and government of the people".
But analysts at cybersecurity firm ThreatConnect who have scoured the site say that its registration and web-hosting data align with a known Russian military intelligence hacker group dubbed Fancy Bear.
The Powell emails were assembled and posted under password protection on Wednesday. BuzzFeed and the Intercept led the way, and reporters found an unvarnished reflection on Election 2016, US war efforts in Iraq, attempts by Powell to disassociate himself from Clinton's use of a private email server when she was Secretary of State, and a plea from one of Obama's former Cabinet members that Powell join the race for president.
Trump receives the brunt of Powell's harsh words, but a 2014 email to a Democratic donor about Clinton is the definition of damned with faint praise: "I would rather not have to vote for her, although she is a friend I respect. A 70-year person with a long track record, unbridled ambition, greedy, not transformational, with a husband still d***ing bimbos at home (according to the NYP)." The reference, presumably is to a story in the New York Post.
But Powell reserved his greatest disdain for Trump.
Of the birther movement, which Trump led earlier this decade by publicly questioning whether Obama was born in the US, Powell said its underpinnings were clear.
"Yup, the whole birther movement was racist," Powell wrote to journalist and former aide Emily Miller on August 21. "That's what the 99% believe. When Trump couldn't keep that up he said he also wanted to see if the certificate noted that he was a Muslim."
In another email to Miller on August 21, Powell scoffs at Trump's apparent effort to reach out to African Americans in recent weeks and says Trump takes them "for idiots". "He is at 1% black voters and will drop. He takes us for idiots," Powell wrote. "He can never overcome what he tried to do to Obama with his search for the birth certificate hoping to force Obama out of the presidency." Powell endorsed Obama in both of his campaigns.
He expressed sympathy for Clinton about repeated investigations by congressional Republicans into the deadly attacks on the US outpost in Benghazi, Libya.
He called it a "stupid witch hunt" in an email to his successor at State, Condoleezza Rice.
But he bristled at attempts by Clinton and Democratic officials to compare his use of private emails while in the office to her use of a private server.
He wrote in May to Democratic consultant Vernon Jordan that he had told Clinton's "minions repeatedly that they are making a mistake trying to drag me in, yet they still try". "The media isn't fooled and she is getting crucified. The differences are profound and they know it."
In another email he added: "Everything [Clinton] touches she kind of screws up with hubris."
On other subjects, the 2003 invasion of Iraq and subsequent occupation haunted Powell.
He and Rice exchanged several emails in which they disparaged former Defence Secretary Donald Rumsfeld.
On June 9 last year, after Rumsfeld claimed he had always been sceptical about US efforts to promote democracy in Iraq after the 2003 invasion, Rice emailed Powell, saying caustically, "If Don and the Pentagon had done their job (after claiming the rights to lead post-war rebuilding - things might have turned out differently). Don should just stop talking. He puts his foot in his mouth every time."
The next day, Powell sent an email praising the "magnificent job" comedian Jon Stewart had done in a skit on Rumsfeld. "Don needs to stay out on the shore," he wrote.
On August 28 this year, Powell turned his ire on Paul Wolfowitz, the Deputy Defence Secretary who was one of the architects of the Iraq invasion, after an interview with Der Spiegel in which Wolfowitz blamed the State Department for bungling the occupation by disbanding the Iraqi army and firing Baathists from government jobs.
"Are you going to call Paul and tell him he is a f***ing liar?" Powell wrote to Larry Wilkerson, his chief of staff. "It wasn't State, it was the President and the guy who reported to Paul ..."
In the last few months, DCLeaks also has posted caches of emails from retired US Air Force General Philip Breedlove, who until recently was the supreme commander of Nato forces in Europe.
It has also leaked material from the Republican Party and George Soros, a billionaire philanthropist who supports liberal causes.
Releasing emails from Powell, however, was seen as an effort to jack up media interest, analysts said.
"They have to find personalities and content which will cause journalists to take interest," said Rich Barger, chief information officer at ThreatConnect, which has analysed the DCLeaks site and links to Russian organisations.
Powell has been hacked before. In 2013, a Romanian hacker known as "Guccifer" hacked his personal email account and released some material, including correspondence that suggested a possible romantic link with a Romanian diplomat. Powell denied having an affair with her.
Included in the latest emails was one from Obama's former Veterans Affairs Secretary Eric Shinseki, who after Trump's Super Tuesday victories wrote to ask Powell to jump into the Republican primary.
"I worry at what is happening to your proud and distinguished Party," Shinseki wrote.
Powell forwarded the message to an aide with a one-word reply: "Sigh." His response to Shinseki was a little more diplomatic.
"C'mon Ric. I would be 80 upon my first months in office," he wrote, adding, "I can't carry the burden of the GOP. They left me years ago."
