Mr Tillerson had used the alias email address while at the oil company to send and receive information related to climate change and other matters.
"It (alias email address) allowed a limited group of senior executives to send time-sensitive messages to Mr Tillerson that received priority over the normal daily traffic that crossed the desk of a busy CEO. The purpose was efficiency, not secrecy," attorneys at the law firm Paul Weiss Rifkind Wharton & Garrison LLP said.
Had Mr Tillerson set up the alias email address with the purpose of "secrecy." emails to the account would have avoided any reference to Mr Tillerson as the intended recipient, the attorneys said.
"Obtaining publicity, not information, appears to have been the real goal of the New York Attorney General's March 13 letter," the attorneys said.
Exxon has provided 2.5 million pages of documents in response to a subpoena from Mr Schneiderman's office.
"More than 16 months after receiving our subpoena, Exxon is just now admitting it may not have preserved or produced the emails of its former CEO, who used an alias email account. We look forward to addressing these, and the other issues our letter raised, in court," Amy Spitalnick, press secretary at the office of the New York attorney general, said in an email.