Solution
To enable the appender:
1. Log into the webadmin interface, using the "admin" account.
2. From the "Domains & Accounts" context, open the "Manage Domains" page
3. Click the "Edit" button next to the domain you need to configure.
4. In the "Message Appender" tab, enable the appender check-box.
5. Type the message to be appended in the text-box below.
6. Click the "Save Configuration" button to make the configuration permanent.
NOTE: The message configured will be appended to ALL messages sent from that domain.