The ability to see east-west traffic is its most valuable feature. Traditionally, email defense focuses on north-south, inbound-outbound, egress-ingress traffic. With Cisco Secure Email Cloud Mailbox, it's able to quickly identify, track, tag, and categorize emails that are internal. That can typically give us visibility into if there's an internal compromised account (for example). Someone can then use that internal compromised account to email additional accounts with either malicious software or links, but internal within that Office tenant. Effectively, that email message never leaves the tenant. Any of the mail gateways really do not have any method or way of seeing this traffic since it's not leaving the environment.
