The best way to prevent the session during the initial TCP communication is to configure policies to stop and reject communication from the known malicious domain. This will prevent the ESA from accepting any messages from that domain and send a negative SMTP response code back to the sender. This will also save the ESA's resources and bandwidth, as it will not have to process or store the malicious emails. This can be done by creating a sender group in the Host Access Table (HAT) that matches the malicious domain and setting the mail flow policy to "Reject" or "Throttle". Alternatively, a message filter can be created that checks the envelope sender against the malicious domain and applies the "stop_connection" or
"reject_connection" action12.
The other options are not as effective as stopping and rejecting the communication at the TCP level.
Configuring the Cisco ESA to drop the malicious emails (option A) will still allow the ESA to accept the messages and then silently discard them, which will consume the ESA's resources and bandwidth, and also not notify the sender of the rejection. Configuring policies to quarantine malicious emails (option B) will also require the ESA to accept and store the messages, which will take up disk space and require manual or automated management of the quarantine. Configuring the Cisco ESA to reset the TCP connection (option D) will abruptly terminate the connection without sending a proper SMTP response code, which may cause the sender to retry the delivery and generate more traffic. Resetting the TCP connection is also considered a less polite and less compliant way of rejecting messages than sending a negative SMTP response code34. References: 1: How to Block a Sender Domain on the Email Security Appliance 2: Message Filters on the Cisco Email Security Appliance 3: How to Configure the Cisco Email Security Appliance to Reject or Drop Messages 4: Cisco Email Security Appliance User Guide - Configuring Mail Policies