One way to protect administrator mailboxes is not to use them. And if you want administrators to use separate mailboxes for their permissioned and non-permissioned activities, that’s what you might do. However, we can be smarter and use transport rules to selectively block email sent to administrator mailboxes to dissuade internal people from sending email and blocking all but the most essential email coming in from external domains.
How to use Exchange Online mail flow rules to improve the effectiveness of your EOP spam filtering and protect Office 365 users from phishing attacks.
How to use mail flow rules in Exchange Server and Exchange Online to block outbound email to specific domain names.
How to use an Exchange Server transport rule to redirect emails that are being sent to the wrong address.
Why using transport rules for email signatures and disclaimers may not give you the results you’re hoping for.