Nonetheless, investigators believe that the exchanges, which reflect Manafort's willingness to profit from his prominent role alongside Trump, created a potential opening for Russian interests at the highest level of a US presidential campaign, according to people familiar with the probe.
Several of the exchanges, which took place between Manafort and a Kiev-based employee of his international political consulting practice, focused on money that Manafort believed he was owed by Eastern European clients.
The notes appear to be written in deliberately vague terms, with Manafort and his employee, Konstantin Kilimnik, never explicitly mentioning Deripaska by name.
Investigators believe that key passages refer to Deripaska. The billionaire is referenced in some places by his initials, "OVD," and one email invokes an expensive Russian delicacy in what investigators believe is a veiled reference to Manafort's past work with Deripaska.
In one April exchange days after Trump named Manafort as a campaign strategist, Manafort referred to his positive press and growing reputation and asked, "How do we use to get whole?"
Manafort spokesman Jason Maloni said today that the email exchanges reflected an "innocuous" effort to collect past debts.
"It's no secret Mr Manafort was owed money by past clients," Maloni said.
Maloni said no briefings with Deripaska ever took place, but that, in his email, Manafort was offering what would have been a "routine" briefing on the state of the campaign.
Vera Kurochkina, a spokeswoman for Rusal, the company led by Deripaska, today derided inquiries from the Washington Post that she said "veer into manufactured questions so grossly false and insinuating that I am concerned even responding to these fake connotations provides them the patina of reality".
The email exchanges add to an already perilous legal situation for Manafort, whose real estate dealings and overseas bank accounts are of intense interest for Mueller and congressional investigators as part of their examination of Russia's 2016 efforts.
People close to Manafort believe Mueller's goal is to force the former campaign chairman to flip on his former Trump associates and provide information.
In August, Mueller's office executed a search warrant during an early morning raid of Manafort's Alexandria, Virginia, condominium, an unusually aggressive step in a white collar criminal matter.
Mueller has also summoned Maloni, the Manafort spokesman, and Manafort's former lawyer to answer questions in front of a grand jury. Last month, Mueller's team told Manafort and his lawyers they believed they could pursue criminal charges against him and urged him to cooperate in the probe, providing information about other members of the campaign. The New York Times reported this week that Manafort had been threatened with indictment by prosecutors.
The emails now under review by investigators and described to the Post could provide prosecutors with additional leverage.
Kilimnik did not respond to requests for comment. A spokesman for Mueller declined to comment.
Deripaska, one of Russia's richest men, is widely seen as an important ally of that country's President, Vladimir Putin. A US diplomatic cable from 2006, published by WikiLeaks, referred to Deripaska as "among the 2-3 oligarchs Putin turns to on a regular basis".
The billionaire has struggled to get visas to travel to the United States due to concerns he might have organised crime ties in Russia, according to the Wall Street Journal. He has vigorously denied any criminal ties.
Russian officials have frequently raised the matter over the years with US diplomats, according to former US officials familiar with the appeals.
In 2008, one of Manafort's business partners, Rick Davis, arranged for Deripaska to meet then-presidential candidate John McCain at an international economic conference in Switzerland.
At the time, Davis was on leave from Manafort's firm and was serving as McCain's campaign manager. The meeting caused a stir, given McCain's longtime criticism of Putin's leadership.
The Washington Post reported in 2008 that Deripaska jointly emailed Davis and Manafort following the meeting to thank them for setting it up. Davis did not respond today to a request for comment.
At the time of the McCain meeting, Manafort was working in Ukraine, advising a Russia-friendly political party. He ultimately helped to elect Viktor Yanukovych as President in 2010. In 2014, Yanukovych was ousted from office during street protests and fled to Moscow.
Manafort and Deripaska have both confirmed they had a business relationship in which Manafort was paid as an investment consultant. In 2014, Deripaska accused Manafort in a Cayman Islands court of taking nearly US$19 million intended for investments, then failing to account for the funds, return them or respond to numerous inquiries about exactly how the money was used. There are no signs in court documents that the case has been closed.
The emails under review by investigators also show that Manafort waved off questions within the campaign about his international dealings, according to people familiar with the correspondence.
Manafort wrote in an April 2016 email to Trump press aide Hope Hicks that she should disregard a list of questions from the Post about his relationships with Deripaska and a Ukrainian businessman, according to people familiar with the email.
When another news organisation asked questions in June, Manafort wrote to Hicks that he never had any ties to the Russian Government, according to people familiar with the email.
Hicks, now the White House communications director, declined to comment.
Former campaign officials said that Manafort frequently told his campaign colleagues that assertions made about him by the press were specious. Hicks, however, told colleagues she was uncomfortable with Manafort's style and concerned he was not always putting the candidate's interests first.
The emails turned over to investigators show that Manafort remained in regular contact with Kilimnik, his longtime employee in Kiev, throughout his five-month tenure at the Trump campaign.
Kilimnik, a Soviet Army veteran, had worked for Manafort in his Kiev political consulting operation since 2005. Kilimnik began as an office manager and translator and attained a larger role with Manafort, working as a liaison to Deripaska and others, people familiar with his work have said.
People close to Manafort told the Post that he and Kilimnik used coded language as a precaution because they were transmitting sensitive information internationally.
In late July, eight days after Trump delivered his GOP nomination acceptance speech in Cleveland, Kilimnik wrote to Manafort with an update, according to people familiar with the email exchange.
Kilimnik wrote in the July 29 email that he had met that day with the person "who gave you the biggest black caviar jar several years ago," according to the people familiar with the exchange Kilimnik said it would take some time to discuss the "long caviar story," and the two agreed to meet in New York.
Investigators believe that the reference to the pricey Russian luxury item may have been a reference to Manafort's past lucrative relationship with Deripaska, according to people familiar with the probe.
