The union denied wrongdoing, saying it was the victim of a media witch-hunt. Labour insisted it had not seen the emails but acted "swiftly and thoroughly" throughout the affair to uphold party integrity.
Labour's investigation was closed last month when witnesses withdrew evidence claiming they were recruited to the party in a drive by Unite to cram Falkirk Constituency Labour Party (CLP) with its supporters to back Karie Murphy, its favoured candidate, in a selection battle.
But the emails are said to suggest the retraction letter was written by Unite officials and approved by CLP chairman Stephen Deans, a figure at the centre of the dispute who is also the union convenor at Grangemouth petrochemical plant.
Tomorrow, Ineos, which runs the plant - saved last week after a union U-turn to accept a management survival plan - is to publish its report on Mr Deans amid claims of "inappropriate use of company resources and systems" - using company time and equipment for Unite political work. The emails were supposedly obtained by Ineos lawyers probing the claims against Mr Deans.
They are said to include a draft of the retraction letter, sent to him so he could get it signed by Michael and Lorraine Kane, the witnesses who initially complained to the party against Mr Deans and Ms Murphy.
They were suspended by Labour pending the inquiry but the charges against them were dropped and they have been reinstated after the Kanes withdrew their complaint.
The file of some 1000 emails has been handed to Police Scotland. Police were called in by Labour when allegations of irregularities emerged in July but the force concluded there was not enough evidence for a full investigation.
Labour's former general secretary Peter Watt said: "Understandably the initial inquiry was pulled because of lack of evidence. If new information has surfaced that puts a question mark over that lack of evidence the party should consider reopening its inquiry."
Tory chairman Grant Shapps said: "Labour must reopen the Falkirk inquiry immediately. These emails confirm Unite planned to infiltrate the Falkirk Labour Party to ensure their candidate was selected and … went so far as to write the statements from key witnesses withdrawing their evidence."
But the union hit back, stressing both Labour and the police investigated the original rigging claims and found Unite broke neither the law nor party rules.
"The email exchanges, apparently leaked by an employer for its own purposes, do nothing to change that," said a spokesman.
He stressed Unite's probe into Falkirk was done by external solicitors and the union had no direct contact with Labour's probe.
The spokesman said it was normal for those facing allegations to seek legal advice, for Mr Deans to deal with the Kanes as they were "members of his family", and for Unite to help members. He added: "This continuing media witch-hunt demonstrates how threatening some elements in society … find such involvement by working people."
A Labour source said the party has yet to see the emails but will consider "any new material". The Falkirk selection process is expected to resume soon.