It is like a gateway for email. They receive all your email traffic. They send over your email traffic, and it is the first incoming point and the last outgoing point. They deliver the traffic to the destination. Whatever it is, you want to be informed of what is happening. Depending on the site's deployment, if you have a single device, then you have all the information on the device. And if you have several devices, you have all the information on every single device for each device. However, for consolidation, you need another device called Security Management Appliance (SMA).
It has no real interaction with other stuff. It does not interact with a gateway beyond the networking level. You have a router and that router provides IP addresses for a switch, etc. You don't have to integrate Cisco Secure Email with something specific since it is standalone and only requires basic essential networking. You can integrate it with a firewall, like ASA, but that firewall has to allow traffic. To do that, you would open port 25.
It is available to be deployed as on-premises, on the cloud, and hybrid cloud.