We have an employee who left a couple of years ago. His email address email@example.com does NOT exist on our mail server and he no longer has an AD account. A user on another network contacted us saying emails to firstname.lastname@example.org were going to him. The strange thing is this guy's email is the same email@example.com. So, the user portion of the email is the same as the guy who is no longer here. The really strange thing is that my CTO did a test email from INSIDE our network and sent an email to firstname.lastname@example.org expecting a bounceback since that user doesn't exist. BUT, the guy at email@example.com actually got it and responded!
I have no idea how this can be happening or how to find out how to stop it. Why would our Exchange server accept an email for a user that doesn't exist on it and actually send it to another domain?