A zombie is some type of account under control by a remote source without the consent of the account's owner. For example, most computers are infected with some type of RAT virus so that when some hacker gets fed up with a website, he can control all of his zombie computers and flood the server, effectively denying service (see: distributed denial of service attack) to everyone. Zombie email accounts are just email accounts that are under control by a third party and are used to send out emails automatically.