Smoothwall have deprecated their Anti-Spam functionality, which you used to be able to append a footer to emails. Now, a cloud based anti-spam filter, powered by MailRoute has replaced this.
Mailroute don't support modifying the email body in transit - largely to protect the integrity of your email. Doing so would also break any signed emails.
You want to add a footer to your email but neither the Smoothwall or MailRoute have this functionality.
If you're using Microsoft Exchange, use the exchange method, detailed here: https://docs.microsoft.com/en-us/exchange/policy-and-compliance/mail-flow-rules/signatures?view=exchserver-2019
Other email servers can also do this.