The altsrchost command is the most direct and appropriate method for this task. It is used to configure the Sender-Based Routing table on the Cisco Email Security Appliance (ESA). This feature is specifically designed to route outbound email to a designated virtual gateway based on the envelope sender's address. This provides a flexible and centrally managed way to control delivery paths for specific senders or domains, which can be considered "flagged" for special routing. While message filters can also achieve this, altsrchost is the dedicated routing feature for this purpose, making it the most precise and standard method.

A. Set up the interface group with the flag: Interface groups are used to bundle network interfaces for high availability or load distribution, not for conditional routing of individual messages based on flags.

C. Map the envelope sender address to the host: This describes the function of Sender-Based Routing, but it is not the method an engineer uses. The altsrchost command is the specific action taken to create this mapping.

D. Apply a filter on the message: While a message filter using the alt-src-host() action is highly flexible, its primary purpose is policy enforcement. Using the dedicated routing feature (altsrchost) is the more direct and standard practice for sender-based delivery control.

1. Cisco AsyncOS 14.0 for Email Security User Guide

Chapter: "Routing and Delivering Mail"

Section: "Sender-Based Routing".

Quote: "Sender-based routing allows you to specify which virtual gateway to use for delivering mail based on the sender’s address as specified in the envelope. You can use sender-based routing to route mail from specific senders to a smarthost for archiving or to route mail from certain domains through a particular Virtual Gateway address." This section details the use of the altsrchost table (managed by the altsrchost command) as the primary feature for this function.

2. Cisco AsyncOS 14.0 for Email Security User Guide

Chapter: "Using Message Filters to Enforce Email Policies"

Section: "Message Actions"

Subsection: "Control Actions".

Reference: This section describes the alt-src-host() action. The context of the entire chapter is policy enforcement

distinguishing its purpose from the core routing functionality described in the "Routing and Delivering Mail" chapter. This supports the choice of the dedicated routing feature (altsrchost) for a routing task.

3. Cisco Content Security Virtual Appliance Installation Guide

Section: "Virtual Gateway".

Reference: This document defines a Virtual Gateway as a combination of an IP interface and a hostname on the appliance used for mail delivery. The altsrchost command or alt-src-host() filter action directs mail to one of these configured Virtual Gateways.