You need to add the label "External" to the subject line of each email message received by your organization from an external sender

Your company has an Office 365 subscription. You need to add the label “External” to the subject
line of each email message received by your organization from an external sender.
What should you do?

A.
From the Exchange Control Panel, add a MailTip.

B.
From the Forefront Online Protection Administration Center, set the footer for outbound email.

C.
Run the Enable-InboxRule cmdlet.

D.
From the Exchange Control Panel, run the New Rule wizard.