The RAT (Recipient Access Table) is the list that contains the allowed recipient addresses on the Cisco ESA.
The RAT controls whether the ESA accepts or rejects messages to a recipient address based on the sender group and the mail policy. The RAT can be configured to allow, relay, or reject messages to specific recipients or domains. The RAT can also be used to apply different message filters or content filters to different recipient groups12. References: 1: Implementing and Operating Cisco Security Core Technologies (SCOR) course, Module 2: Network Security, Lesson 3: Deploying Cisco Email Security Appliance, Topic: Configuring Mail Policies2: Cisco Email Security Appliance User Guide, Release 13.5 - Configuring Mail Policies [Cisco Secure Email Gateway] - Cisco.