Fortimail is a dedicated Email Security appliance which you can use for Exchange server, Office 365, Google APP or other vendors as well. This will scan your emails before delivering to users inbox and stop spams, malware to deliver to the user's mailbox.
Fortimail is working on an advanced level as it will check authentication, RLB, DKIM, DMARC, SPF, Sandboxing etc.
Now FortiGate email filtering with only a few options (10% of actual Mail security) and it is working between User's Mailbox and Mail Client as Outlook. But same time if it is working on an encrypted protocol then it is very difficult to scan it.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.