System accounts are specific accounts used by ZCS. They include the spam filter accounts for junk mail (spam and ham), virus quarantine account for email messages with viruses, and GALsync account if you configure GAL for your domain. These accounts do not count against your license.
Do not delete these accounts!.
To see a list of system accounts
Click Search in the Navigation pane to go to the Advanced search page.
In the Search Option section, click Basic Attributes.
Check Search system accounts only.
Click OK. The list displays in the content pane.