Jackson, in a brief written statement, said Thursday she is leaving the EPA after four years on the job, for "new challenges, time with my family and new opportunities to make a difference."
The agency did not offer an explanation. But Chris Horner, a senior fellow at the Competitive Enterprise Institute, said the scrutiny over the alias emails is clearly a factor.
"Life's full of coincidences, but this is too many," he told FoxNews.com. "She had no choice."
Horner and CEI earlier this year had sued the EPA for documents pertaining to Jackson's use of alias email accounts. She was said to operate under the name "Richard Windsor" -- the use of those accounts has since drawn the scrutiny of Republican members of Congress, as well as triggered an audit by the EPA inspector general.