Scroll to the Content compliance setting in the Compliance section, hover over the setting, and click Configure. If the setting is already configured, hover over the setting and click Edit or Add another
Check the boxes next to the messages you want the rule to apply to. In this specific scenario, choose OUTBOUND.
Go to the next step to continue.
From the list, specify whether any or all conditions must match to trigger what happens to the message. For example, if you select If ANY of the following match the message, any matching condition can trigger the consequence to the message.
Choose Advanced content match, where on Location, choose Full Headers and Match Type, choose Matches Regex. On Regexp, add the following expression:
IMPORTANT: Note that ''10'' is the allowed amount of email addresses that are allowed to be entered in the BCC field. Once the expression is added, please click on Save.
Specify whether to modify, reject, or quarantine a message when conditions are met.