When users are no longer working on your domain, you can remove their mailbox and service accounts to conserve resources and maintain security.
Note: A special alias called site_blackhole is automatically created when the domain is created. It is not shown in the list of aliases and its only purpose is to act as a repository for all garbage email.
When a user with an alias is removed, the alias can still remain (as it may be referenced by other aliases), but the target user name is replaced by site_blackhole. When an email message is sent to this alias, it will go to site_blackhole. Therefore, when you delete a user, you will see the alias set to site_blackhole.
To remove a user from a domain: