Those emails are under review by the FBI, and their content has not been released publicly, but Trump appears certain in his pronouncement despite a lack of evidence.
His trading on uncertainty without proof may seem familiar: His talk of the renewed probe mirrors his years-long perpetuation of the birther movement.
During Trump's initial push of birtherism, he repeatedly suggested that there was something amiss with Obama's birth certificate, despite having no evidence to support that claim.
"I want him to show his birth certificate," Trump said on ABC's "The View" in 2011. "There is something on that birth certificate that he doesn't like."
Birtherism was strongly linked to the theory that Obama is a secret Muslim and wants to hide his religion from the American people. Trump floated that possibility on Bill O'Reilly's show on Fox News in 2011.
"People have birth certificates. He doesn't have a birth certificate. He may have one, but there's something on that, maybe religion. Maybe it says he is a Muslim," he said. "I don't know. Maybe he doesn't want that. Or he may not have one. But I will tell you this. If he wasn't born in this country, it's one of the great scams of all time."
Trump also claimed on the "Today" show in April 2011 that he dispatched investigators to look into the matter "and they cannot believe what they're finding."
He claimed during the segment that Obama's certificate of live birth was "easy to get" and that Obama spent $2 million in legal fees to avoid the issue.
That month, Obama released his birth certificate, but that still didn't satisfy Trump, who wrote on Twitter that it was a "fraud."
And in 2013, one of the last times that he publicly discussed the issue before his presidential run, Trump again refused to acknowledge the evidence presented by Obama.
"Well, I don't know. Was there a birth certificate? You tell me. You know some people say that was not his birth certificate. So maybe it was, maybe it wasn't," he told ABC News' Jon Karl.
As recently as Sept. 15 of this year, Trump refused to acknowledge that Obama was born in the United States, but the next day, after five years, Trump finally conceded, "President Barack Obama was born in the United States. Period."
Clinton Email Probe
Trump has gone to great lengths to paint Clinton as the most corrupt politician in American history, repeatedly calling her "crooked Hillary," among other monikers.
"Hillary's corruption is a threat to democracy, and the only way to stop it is for you on Nov. 8 to show up at the polls and vote," he said at an appearance in Warren, Michigan, Monday.
He even threatened during their second debate to throw her in jail if he becomes president.
Clinton was admonished this summer by FBI Director James Comey for her handling of email, including classified information, on her private email server while she was the secretary of state. She has acknowledged she made "a mistake" by using the private server.
But she and her top aides have not been criminally charged in the matter. The FBI did not recommend charges in the case, and the DOJ declined to prosecute.
Comey explained at the time that "it takes mishandling it and criminal intent" to make a case.
"I think she was extremely careless. I think she was negligent. That I could establish. What we can't establish is that she acted with the necessary criminal intent," he said. "'Should have known,' 'must have known,' 'had to know' does not get you there. You have to prove beyond a reasonable doubt that they knew they were engaged in something that was unlawful."
In a memo to FBI employees, Comey said the case was "not a cliffhanger" and "really wasn't a prosecutable case."
In a letter to lawmakers Friday, Comey said the new emails "appear to be pertinent to the investigation." But in an internal memo sent later that day, he said the agency doesn't "know the significance of this newly discovered collection of emails."
Clinton has slammed the review, saying there is "no case here."
But that didn't stop Trump from pouncing on Monday. "We can be sure that what is in those emails is absolutely devastating, and I think we're going to find out, by the way, for the first time," he said.