Exxon Mobil confirmed the secondary email account and stressed that it was not used exclusively for climate change matters, saying in a statement it was used for "a broad range of business-related topics."
Company spokesman Alan Jeffers said the account was "mostly used while he [Tillerson] was traveling to ensure he'd see an item in a secure and timely way without having to sort through his regular email inbox, which could contain many items. To suggest it was exclusively or even primarily about climate issues is absolutely false and misleading."
The state attorney general's office discovered the Wayne Tracker account during an investigation of Exxon Mobil to determine whether the company misled shareholders and the public about possible financial risks that climate change poses to its business.
The investigation began in November 2015, and on Monday the attorney general's office sent a letter to New York County Supreme Court saying Exxon Mobil "has failed to comply in good faith" with a subpoena issued to produce documents from the company's top executives, including from the Wayne Tracker email account.
"Despite the company's incidental production of approximately 60 documents bearing the 'Wayne Tracker' email address, neither Exxon nor its counsel have ever disclosed that this separate email account was a vehicle for Mr. Tillerson's relevant communications at Exxon, and no documents appear to have been collected from this email account, which also does not appear on Exxon's list of preserved custodial sources or its privilege logs," the letter reads.
The letter also says there are "34 additional email accounts specifically assigned to top executives, board members or their assistants" that were not initially produced in the investigation.
Exxon Mobil said in a statement that the company "has been responsive … and has provided more than 2.5 million pages of documents."
"Media reports indicating that emails to or from this address were exclusively for climate-related topics are false," the statement said. "The very fact the attorney general's office has these emails is because they were produced in response to the subpoena."
It continued, "Rex Tillerson's main email address received a large volume of messages from a variety of internal and external sources. The email address Wayne.Tracker@exxonmobil.com is part of the company's email system and was put in place for secure and expedited communications between select senior company officials and the former chairman for a broad range of business-related topics."
It is not unheard of for business executives to maintain alias email accounts for inner circle communications.
The company also said it "believes the risk of climate change is clear and warrants action."
Tillerson's middle name is Wayne, but the company did not explain the origin of the name Wayne Tracker.
"I'm not sure how the name was chosen," Jeffers told ABC News in an email.
The news of the secondary email account has raised questions among climate advocacy groups.
"If they had nothing to hide," asked Jamie Henn, the communications director at climate change advocacy group 350.org, "then why the secret email account?"
State Department deputy spokesman Mark Toner said on a call with reporters today that in his current role, Tillerson “uses only his Department of State email address to conduct official business and he does comply with all federal record-keeping requirements.”
“I can assure you, very disciplined about that,” Toner said of Tillerson’s use of his official email address. “Similarly, he only uses an official phone number for conversations he may have with respect to his business. I am simply unaware that he might have – or whether he has – a personal email address in addition. But in the conduct of official business, he only uses a Department of State email address.”