REUTERS/Pablo Martinez Monsivais/Pool Putin is very keen on knowing as much as possible about his adversaries.
What makes the intrusion troubling is the content of those emails.
Officials conceded to the Times that, "the unclassified system routinely contains much information that is considered highly sensitive: schedules, email exchanges with ambassadors and diplomats, discussions of pending personnel moves and legislation, and, inevitably, some debate about policy."
The exact content of what the hackers took is being kept secret - so the Russians aren't tipped off to what is known - and people close to the president said that he is careful about emailing outside of the White House system.
Nevertheless, it's clear from the reporting that the Russians learned some things about the commander in chief.
"Obama is known to send emails to aides late at night from his residence, providing them with his feedback on speeches or, at times, entirely new drafts," the Times reports. "Others say he has emailed on topics as diverse as his golf game and the struggle with Congress over the Iranian nuclear negotiations."
The New York Times said the breach had been "far more intrusive and worrisome than has been publicly acknowledged."
One senior American official briefed on the investigation told the Times, "This has been one of the most sophisticated actors we've seen."
Another senior official added that it's "the Russian angle to this that's particularly worrisome."
Reuters President Obama displaying his Blackberry to reporters after nearly leaving it behind before of a flight to Las Vegas on November 21, 2014.
The Times notes that the president's email account itself and his encrypted Blackberry were apparently not hacked.
Officials did not disclose the number of Obama's emails that were read by the hackers.