I mean, depends on the email. If you spend more time answering yourself than the AI would, you almost certainly emit more green house gasses, used more fresh water and electricity, and burned more calories. Depending on the email, you might have also decreased net happiness generally.

Do we care about the environment or not? Please, oppose datacenters in desserts and stop farming alphalpha where water supplies are low. But your friend using AI to answer an email that could have been a google search is not the problem.