Speaking to reporters this morning, Mr Key said he had discussed the issue with Mr Whitney, who works at Auckland-based firm Antipodes Trust Group.
"He is absolutely confident my version of events is correct," he said.
"Maybe the email was sloppily written. I was certainly aware that we weren't making any changes."
The Prime Minister was asked how an experienced lawyer had misinterpreted an important conversation.
Mr Key said: "That's his wording of it. People sometimes write things in a way that is shorthand."
Mr Whitney has not responded to requests for comment.
Labour leader Andrew Little was sceptical of Mr Key's recollection of events. He said Mr Whitney was a trained lawyer and "knows about language".
The exchanges between Mr Key and Mr Whitney had "clearly resulted in the IRD backing off on a review of foreign trusts", Mr Little said.
At the time the email was sent, Mr McClay was a new minister.
Labour and Greens say that Mr Whitney's use of the Prime Minister's name would have put pressure on the minister.
Around five months later, the minister told the foreign trusts industry that the IRD would not be carrying out a review.
Revenue Minister Michael Woodhouse downplayed any connection between Mr Whitney's lobbying and the IRD's decision to drop the review of foreign trusts.
He said the industry had been concerned about possible taxation of foreigners who held assets or money in trusts in New Zealand.
"That was not the case," Mr Woodhouse said. "There was no intention to change that."
The IRD was also looking at whether offshore investors who held trusts in New Zealand should have to disclose more information.
Tax officials said they were not prepared to drop any work to review the disclosure rules, Mr Woodhouse said.
The minister said that he had never been lobbied by Mr Whitney, but he had spoken to members of Mr Whitney's lobby group.
