That is the part that smells to me too. I don't know any CEO, manager, or person otherwise responsible for employees, that would take a single resignation email as the one and only thing to start the paperwork and rehire a replacement.

The cynic in me, say that the CEO was unhappy with performance and/or the leave. Used his posistion to gain control and send the mail to get the ball rolling.

If it wasn't him, I would fully expect him to launch an investigations right then and there when it became apperant that someone spoofed an email. IT should have all the logs necessary to figure out where the email was send from.